Canadian Railroad signals

https://www.youtube.com/watch?v=tJpR93kp44I Part 1
https://www.youtube.com/watch?v=16jXTDfEavA Part 2
https://www.youtube.com/watch?v=b4lUiaWT8wE Part 3
https://www.youtube.com/watch?v=Oqq0B0Q9exk Part 4

North American Railway Signaling

From Wikipedia, the free encyclopedia

For the actual physical signals, see North American railroad signals.

Standards for North American railway signaling in the United States are issued by the Association of American Railroads (AAR), which is a trade association of the railroads of Canada, the USA, and Mexico. Their system is loosely based on practices developed in the United Kingdom during the early years of railway development. However, North American practice diverged from that of the United Kingdom due to different operating conditions and economic factors between the two regions. In Canada, the Canadian Rail Operating Rules (CROR) are approved by the Minister of Transport under the authority of the Railway Safety Act. Each railway company or transit authority in Canada issues its own CROR rulebook with special instructions peculiar to each individual property. Among the distinctions are:

• The US has a much longer history of power operation of switches ("points" in UK parlance) and signals.
• The US and Canada departed from UK practice wherein a semaphore blade is devoted to each route (Route Signaling). North American practice is to group routes by speeds and use a single blade for, say, "medium speed" regardless of the number of routes involved (Speed Signaling). The primary exception to this situation is in the field of heavy rapid transit, such as subways and elevated lines, which make use of a localization of the British practice, or a combination of the AAR and British systems.

Contents

  [hide] 

• 1 History of railroad operating rules
• 2 Historic methods of operation
  • 2.1 Timetable operation
  • 2.2 Timetable and train order
• 3 Modern signaling in the U.S. and Canada
• 4 North American Train Control Systems
  • 4.1 Restricted speed operation
  • 4.2 Absolute or manual block
  • 4.3 Basic Automatic Block Signals
  • 4.4 Signaling enhancements
• 5 Effect of mergers on signaling and operating rules

History of railroad operating rules[edit]

In railroad operations, nearly every aspect of employee behavior is governed by operating rules. Employees who perform their jobs in an unsafe manner usually violate operating and safety rules. Human-factors related incidents are caused, or influenced by, unsafe work behavior and attitudes, as opposed to non-behavior related factors like inclement weather or undetected faulty track. Therefore, nearly all human-factor incidents and injuries can be associated with one or more operating or safety rule violations.^[1][2]

By the 1850s, railroad operating rules, often printed as pamphlets or on the back of a time card, had evolved to near universal application. On April 14, 1887 representatives of 48 railroads voted for the adoption of what is now known as the Standard Code of Operating Rules (SCOR), published by the AAR. Thus, all railroad rule books in North America today have as their foundation the SCOR in both development and application.^[3]

The SCOR, however, was never intended to be used as a working rulebook. Rather, its primary intention was to standardize operating practices to the extent practicable while still preserving the flexibility of individual railroads to either modify or omit rules at their discretion. Even rulebooks with identical phraseology could be interpreted and applied differently on different railroads. Although used as a reference book, the SCOR was primarily a matrix document, from which the industry could establish standard text and a common numbering system. Until recently, in fact, railroads rarely deviated from the original numbering system.^[4]

At present, most Class I railroads in the U.S. use one of two “standard” rulebooks: the Northeast Operating Rules Advisory Committee (NORAC) rulebook and the General Code of Operating Rules (GCOR). Conrail, Amtrak, and several commuter and short line railroads in the northeastern United States use the NORAC rulebook. The GCOR is used by every Class I railroad west of the Mississippi River, most of the Class II railroads, and numerous shortline railroads. A few railroads, including CSX, Norfolk Southern,Illinois Central, Metro North and Florida East Coast, have adopted their own rulebooks. In the case of NS and CSX, the NORAC Rulebook was integrated into their existing rulebook structure with the Conrail merger. Metro-North uses a rulebook based on NORAC. The Long Island Rail Road (LIRR) still uses a rule book that is based on the Standard Code of Operating Rules. Canadian railways use the CROR.

Research indicates unsafe work behavior can be influenced by any number of factors, including temperature, workload, time of day, and specific job tasks to name a few.^[5]Unsafe work behavior has also been linked to the organizational culture and to organizational processes.^[6] Consequently, before unsafe work behaviors in railroad operations can be reduced, some of the reasons employees do not comply with railroad operating rules must be understood. Also, there must be an understanding of some of the cultural components of rule compliance within the railroad system, particularly the development of railroad rules, and how that culture interacts with unsafe work behavior.

Historic methods of operation[edit]

Timetable operation[edit]

All trains must have authority to occupy a main track. Originally possession of a timetable which showed a schedule for that train, authorized a train crew to take their movement onto the main track.

The simplest form of operation, in terms of equipment at least, was operation according to a schedule. Everything was laid down in advance and every train crew knew the timetable. Trains could only operate according to the scheduled time at each station for their train, during which they had exclusive "possession" of the track and no other train could operate on the same track. Trains typically were required to be "clear" of the mainline within five minutes of an opposing or following train's time at the next station. The timetable also specified (usually prominently on the first page) the superiority of trains by direction or class, for example, on most railroads an Eastward train was superior to a Westward train, and a First Class (passenger) train would be superior to a Fourth Class train (freight.) A superior train would normally hold the main track at meeting places, while the inferior train would take the siding on single track territory.

When trains were operating in opposing directions on a single-line railroad, meets were scheduled, where each train waited for the other at a point they could pass. Neither was permitted to move until the other had arrived.

The timetable system had several disadvantages. The first was that there was no positive confirmation that the track ahead was clear; only that it should be clear. This system did not allow for breakdowns and other such problems. The timetable was set up in such a way that there should be sufficient time between trains for the crew of a broken-down or delayed train to walk back up the line far enough to set up warning flags, fusees and explosive devices known as torpedoes, which alerted a train crew to an occupied track ahead.

The second problem was the timetable system's inflexibility; trains could not be added, delayed, or rescheduled without publishing and distributing a new timetable. Trains all down the line might have to stay clear of the main line, waiting for a train which never started from its initial station.

The third was a corollary of the second; the timetable system was inefficient. To give a little flexibility, the timetable gave trains a broad swath of time to allow for some delay. Thus, the line was possessed by the train for much longer than was really necessary.

Nonetheless, this system permitted operation on a vast scale, with no requirements for any kind of communication that travelled faster than a train. Timetable operation was the normal mode of operation on North American railroads in the early days.

Timetable and train order[edit]

With the advent of the telegraph, a more sophisticated system became possible, since the telegraph provided the first system available where messages could be transmitted faster than the trains themselves. The telegraph allowed the dissemination of alterations to the timetable, known as train orders. These overrode the timetable, allowing the cancellation, rescheduling and addition of trains, and almost anything else. Sufficient time had to be given, however, so that all train crews could receive the changed orders.

Train crews generally received the orders at the next station at which they stop, although sometimes orders were handed up to a locomotive "on the run" via a long staff or hoop. Train orders allowed train dispatchers to set up meets at sidings, force a train to wait at a siding for a priority train to pass from behind, and to keep at least one block spacing between trains going the same direction. Train orders could also reverse the superiority of trains, or give extra or inferior trains rights over superior trains in order to accommodate abnormal operating conditions.

In North American railway traffic control, a Form 19 or 31 train order would modify their schedule. An order which did not require a train to stop was called a "Y" (yellow) train order. The operator at the station would display a Yellow signal, which would cause the train to slow down so that they could receive the train order from a hoop from the operator. These commonly gave crews information about track speed (in areas where tracks and bridges needed repair) and were called "slow orders" by train crews. An "R" (red) order told the station operator to display a red (stop) signal, and dealt with meets, waits and other important traffic control issues, it was necessary for the crews to stop because they often had to sign for them, indicating that they had read and understood the situation. Orders were flagged at train stations by telegraph operators who signaled using a fixed "order board", which usually consisted of a single semaphore blade mounted over the operator's position in the station. The operator usually had to confirm to the dispatcher that the requested Y or R signal had been displayed properly before the dispatcher could issue further instructions to other trains or engines concerning that movement.

Timetable and train order operation was commonly used on American railroads until the 1960s, including some quite large operations such as the Wabash Railroad and theNickel Plate Road. Train order traffic control was used in Canada until the late 1980s on the Algoma Central Railway and some spurs of the Canadian Pacific Railway. On CN'sDeux-Montagnes commuter line, this system lasted on part of the route until the total replacement of signaling and catenary in 1995. The orders were notable for being bilingual.

Timetable and train order was not used widely outside North America and has been phased out in favor of radio dispatching on many light-traffic lines and electronic signals on higher-traffic lines. The only railroads currently still using authentic train order operations is the South Shore line in Indiana and the LIRR in New York.

Modern signaling in the U.S. and Canada[edit]

U.S. railroads have historically used a far greater variety of signaling systems than other countries. There have never been national standards for signal appearance and operation, so each of the hundreds of rail lines developed its own signaling techniques.

As Trains magazine describes:

This was no problem as long as crews stayed on home territory. But as roads merged, split, and spun off new short lines, and tenant operators such as Amtrak and regional commuter systems came into existence, train crews could find themselves on several different properties in the course of a work week.

The creation of Conrail in 1976 out of the remains of a half-dozen bankrupt railroads only made things worse. It was hard enough to rationalize the systems of the constituent companies, let alone interact with other operators over the dense Northeastern U.S. rail network.

After several years, the situation had become intolerable. Training costs were getting out of hand, because crews had to qualify separately on each road over which they might operate. Having to consult half a dozen rulebooks increased enormously the potential for a disastrous mistake.^[7]

As railroad companies eventually began to standardize their rule books via industry-wide committees, the implementation of signal systems between railroads became, if not standardized, at least more similar. Different legacy systems still in use, however, mean that some signal indications can be shown in several different ways.

Within the United States, each railroad operator formulates its own operating practices, subject to the regulations in Title 49 Part 236 of the Code of Federal Regulations). However, there are two major groups of railroads that have adopted common operating practices and therefore a common operating rule book. Major railroads on the East Coasthave adopted the NORAC rules. Most railroads west of the Mississippi River, as well as the U.S. operations of the Canadian Pacific Railway, use the GCOR. Some large U.S. railroads, including CSX, Norfolk Southern, and the U.S. portion of the Canadian National do not subscribe to either NORAC or CROR and still use their own rule books.

The NORAC rule book illustrates all signal aspects and indications which may appear on track operated by member railroads. However, GCOR does not illustrate signal aspects and indications because of the lack of uniformity between the participating railroads. Signal aspect and indication illustrations instead appear in each railroad's system special instructions or operating timetable for the region or division where the aspects and indications apply. This practice is necessary due to the lack of uniformity in aspects between the multitude of railroads participating in GCOR, which includes a number of large systems created through merger.

All railroads operating within Canada, including Canadian Pacific, Canadian National, and up until recently, BC Rail, use the Canadian Rail Operating Rules (CROR). These rules are discussed within the forum of the Railway Association of Canada, which makes recommendations for changes to the Minister of Transport of Canada who then approves the rules issued by each company. Canadian Rule Books contain all hand signals, voice signals and flag signals as well as fixed trackside signal indications necessary for operation.

These rule books specify various methods of operation in both signaled territory and dark territory, where manual methods of granting track authority must be used.

North American Train Control Systems[edit]

In North America, train operation over any specific section of track is governed by a specific set of rules in the railroad company's Rulebook. The rules and sections differ from company to company, but they all cover the same basic modes of operations.

Restricted speed operation[edit]

Restricted speed or line of sight operation implies that trains will control themselves so that they can come to a stop within half the range of the engineer's vision or line of sight. There are many types of Restricted speed operations with slight variations, such as Yard Limit rules or Industrial Track rules depending on specific operational circumstances. They all have the same basic working theory, that two trains, approaching head on, will each be able to come to a complete stop upon seeing one another. Restricted speed operation generally works with a maximum speed of 15 or 20 mph and a typical speed governed by length of train and visual conditions.

Absolute or manual block[edit]

Manual block systems work by dividing up a rail line into predefined "blocks", which are typically demarcated by fixed signs. Authority to occupy a block is granted by some sort of central controller, usually a dispatcher, who has the sole authority to grant such access. Before the widespread adoption of radio communications on trains, authority to occupy a block, along with other instructions, were communicated to trains much as timetable train orders were with station agents and trackside telephones. Some railroads, notably thePennsylvania Railroad (PRR), had a system of manual block signals activated by wayside operators in stations or interlocking towers eliminating the need for some trains to stop.^[8] This manual block system is still on use on the Long Island Rail Road, which had been a subsidiary of the PRR.

With the introduction of radio communications, this information could be directly transmitted to the crews. There are several different flavours of this system, but they all share in common a system of blocks, a safe way to grant access to those blocks and a standardized system of paperwork to eliminate confusion for both the crews and the dispatcher.

These types of manual signaling systems typically fulfill the FRA's "signaling system" requirement for trains to exceed 60 mph.

Here are some of the more common systems.

• Track Warrant Control

In Track Warrant Control, or TWC, the train dispatcher issues "track warrants" via radio that authorize the train between two specified limits. The limits are often mileposts or stations. The track warrant may authorize a train to proceed to a station and "clear the main", or enter a siding so an oncoming train can pass. Generally, no more than one train or piece of equipment may be given the same or overlapping limits of authority, unless the movements will be made at restricted speed. While TWC is generally used in dark territory, it may be combined with ABS to enable more than one train to be given the same authority (although this generally only applies to trains moving in the same direction). This reduces the workload of the dispatcher and train crew, as new track warrants do not need to be copied every few minutes to ensure that following trains are not delayed due to running out of authority. In Canada, a Clearance is the equivalent to a Track Warrant.

• Direct Traffic Control

Direct Traffic Control, or DTC, is similar to TWC, except that the rail line is divided up into predefined blocks—somewhat similar to ABS blocks without the signals—and dispatchers authorize trains to proceed in a specified number of blocks. Only one train may occupy a stretch of authority (which may consist of a single block or a stretch of dozens) at any given time, unless movements are to be made at restricted speed. Like TWC, DTC may be combined with ABS in high-traffic areas to aid with train separation and safety.

• Form D Control

Form D Control System, or DCS, is a system similar to Track Warrant Control that is used by railroads subscribing to NORAC (Track Warrant Control is a GCOR term). The name comes from the form that train crews copy the authority on. A sample Form D is available here; line two is used to grant authority for occupying the track.

• Clearance Card Form K Control

This is a form commonly used on railroads such as the LIRR which base its rules on the SCOR. At locations where a "Block-Limit Signal" is displayed in place of a fixed manual block signal, train crews obtain a Clearance Card Form K over the radio or telephone from the operator in charge of that territory, which allows that train to pass that Block-Limit as though it displayed a "Clear-Block". A Clear Block is a condition where a particular block or predetermined length of track is clear of all trains. In addition to the condition of the block ahead, The Standard Code of Operating Rules, such as those still in use on the LIRR, gives one train "superiority" over "inferior" trains. The "superior" train has Right-of-Track over the "inferior" trains. To move the "inferior" train against a "superior" train, Train Orders are used to govern the movement. On this railroad, a compilation of Train Rules such as superiority, and Block Rules such as a "Clear-Block" is used to proceed on the main track.

• Centralized Traffic Control (CTC)

In Canada, fully signaled lines use CTC, where signal indication is the authority for movement. This system is supervised by a Rail Traffic Controller (RTC) who is in constant communication with the trains and engines on his territory. The RTC has the ability to line routes and give permissive signal requests to the system in the field from a central location. The RTC can also issue special permissions to trains via radio. In order to pass signals set at stop (Rule 564), reverse direction within a block (Rule 577) or enter the main line at a manual switch not equipped with a signal (Rule 568), the train crew must copy the RTC's instructions and repeat them back correctly before being allowed to proceed.

• Occupancy Control System (OCS)

In Canada, non-signaled and Automatic Block Signal (ABS) territories are operated according to OCS rules. The only authority for movement in OCS territory is possession of a Clearance or Track Occupancy Permit (TOP) While the line might be equipped with signals (ABS), these signals are unsupervised and uncontrolled by the Rail Traffic Controller. Their indications are strictly governed by track conditions and the passage of trains, and serve to tell operating crews at what speed to operate, and warn of an occupied track ahead.

Basic Automatic Block Signals[edit]

See Automatic Block Signal

Signaling enhancements[edit]

Cab signaling Systems or CSS (also known as Automatic Cab Signaling/Automatic Speed Control, or ACS), is often used as an overlay for ABS, Rule 251 and CTC. This system provides train crews with information about the next signal indication, even if the signal mast is not visible. Automatic Train Stop, or ATS, systems provide wayside inductors that, when activated, alert the engineer that the train has passed a signal other than Clear and if the signal is not acknowledged the train's brakes will be applied. Automatic Train Control, or ATC, adds in-cab enforcement to these and will apply the brakes if a dangerous situation arises, such as when the next signal is displaying a stop indication but the engineer has not begun slowing the train. Some form of ATS or ATC is required on all U.S. rail lines that operate at 80mph or more.

In the 1990s, Communication-based train control (CBTC) systems started to be used in rail transit systems. These systems utilize radio communications between train and wayside equipment to perform the functions of the signaling system. More recently, CBTC systems have been deployed on mainline railroads, and Interoperable Communications Based Signaling (ICBS) systems are being developed to provide standard system functionality among railroads and suppliers signaling systems.

A further enhancement designed to work in both signaled and dark territory is Positive Train Control, or PTC. This system is an overlay on the conventional methods of operation but also uses satellite-based tracking and computerized radio communication to verify the authority given to the train, current location of the train, the status of the next signal (if any), the position of switches (which will be equipped with a sensor and radio transmitter), and the location of any oncoming trains. As in ATC, if a dangerous situation arises, the system will apply the brakes.^[9][10][11]

Effect of mergers on signaling and operating rules[edit]

Mergers of major railroad companies in recent years resulted not only in the merging of different railroad lines and operating rulebooks, but also in the merging of railroad cultures and operating practices. Superficially, it may appear that most railroads have adopted a common code of operating rules, but major differences still exist in the application, and consequently, the compliance with these operating rules. Moreover, different management styles often clash when organizational cultures merge, as documented in the case of the Penn Central merger in 1968^[12] and the Burlington Northern Santa Fe merger in 1995.^[13] This leaves operating rules managers uncertain as to how specific rules should be applied on their newly formed railroad. Different management philosophies may also influence different compliance standards across railroads.

As railroad operating environments become increasingly complex - from mergers, new technology, and other external forces - operating rules will continue to change in both number and frequency. The number of operating rules and procedures that employees must now commit to memory is substantial. With fewer employees to handle the same workload, individuals may no longer have the time to look up rules when performing their duties, perhaps further complicating both their ability and their desire to comply with these rules.

For years, the FRA has been recommending the standardization of operating rules and practices for cost-effectiveness in both safety and efficiency. In 1992, the American Association of Railroad Superintendents (AARS) convened a special committee, which suggested to its Board of Directors, Executive Council, and membership that “the AARS sponsor a full conference on the standardization of railroad operating rules, practices, and procedures, and that this conference be conducted on the highest level possible, with the full and complete endorsement and involvement of chief operating officers.”^[14] A standard set of operating rules, it was argued, would minimize the confusion that exists when employees operate over joint lines and are governed by two rule books instead of one. Of particular concern are situations where different operating rules govern identical, or similar, signal aspects on different railroads.

Some railroad operating officials believe standardized railroad operating rules would have a positive impact on the railroad industry in fundamental and important ways, including:

• increasing the mobility and ease of transition for both railroad employees and managers when transferring from one railroad to another;
• reducing training costs and operating rule development;
• improving safety practices when railroads and railroad rule books merge; and
• improving the overall railroad delivery system across interchange points, regions, and yards.

In response to pressures for standardization, the railroads governed by both NORAC and GCOR recently hired consultants to rewrite and reorganize their operating rulebooks. Two major benefits are expected from these new versions of the operating rule book: 1) an improvement in the clarity and understanding of operating rules, and 2) an improvement in the ability of an employee to look up unfamiliar operating rules.

The extent to which these and other benefits have been obtained, however, is uncertain. Even if the revised rule books enhance the clarity and understanding of operating rules, other important questions still remain. Given a factual understanding, how well are employees able to conceptually apply the rules? How often do operating employees purposely violate rules, even when they understand them and know how to apply them? What influences operating employees to knowingly violate operating rules? How often do rule violations lead to incidents or injuries that otherwise could have been prevented?

Some railroad operating officials urge that both GCOR and NORAC should be used as the SCOR was originally used. They say operating rule books should be used as a basic guide to standardizing operating practices, while still preserving the flexibility of individual railroads to either modify or omit rules at their discretion. If standard operating rules are not needed, the major question that remains is whether or not a process has been established for maintaining quality in operating rule development. This is especially important with the implementation of constantly changing equipment and train control technology, which will force the need for more rapid rule changes. Therefore, the question that must be asked is whether or not guidelines are needed for the development, writing, testing, application, and representation of operating rules. What kinds of guidelines should be developed, if at all? If necessary, what should be the process for developing those guidelines? For this purpose, a focus group was assembled.

North American Railroad Signals

From Wikipedia, the free encyclopedia

North American railroad signals generally fall into the category of multi-headed electrically lit units displaying speed-based or weak route signaling.^[1] Signals may be of the searchlight, color light, position light, or color position light types, each displaying a variety of aspects which inform the locomotive engineer of track conditions so that he or she may keep their train under control and able to stop short of any obstruction or dangerous condition.

There is no national standard or system for railroad signaling in North America. Individual railroad corporations are free to devise their own signaling systems as long as they uphold some basic regulated safety requirements. Due to the wave of mergers that have occurred since the 1960s it is not uncommon to see a single railroad operating many different types of signaling inherited from predecessor railroads. This variety can range from simple differences of hardware to completely different rules and aspects. While there has been some recent standardization within railroads in terms of hardware and rules, diversity remains the norm.

This article will explain some of the aspects typically found in North American railroad signaling. For a more technical look at how signals actually work, see North American railway signaling.

Signaling aspect systems[edit]

There are two main types of signaling aspect systems found in North America, Speed Signaling and Weak Route Signaling.^[2] Speed signaling transmits information regarding how fast the train must be going in the upcoming segment of track, weak route signaling transmits information related to the route a train will be taking through a junction and it is incumbent upon the engineer to govern the train's speed accordingly. Weak Route Signaling is applied with the term "Weak" because some speed signal aspects may be used in the system and also because exact route information is not typically conveyed, only the fact of a diverging or straight route, each having a predictable range of known speeds.

Typically railroads in the Eastern United States ran speed signaling, while railroads in the west used route signaling, with some mixing of systems in the Midwest and South. This was due to the lower train density in the west combined with generally simpler track layouts. Over time, the route signaling railroads have incorporated segments of speed signaling through merger and have also adopted more speed based aspects into their systems. Of the five major Class 1 railroads in the United States, CSX uses speed signaling, Union Pacific and BNSF use speed enhanced route signaling and Norfolk Southern uses a mix of speed and route signaling based on the original owner of the line. Commuter railroads and Amtrak all use speed signaling where they own or maintain the tracks they run on. Canadian railroads all use a strong system of speed signaling in Canada, but have some segments of route signaling on lines they have acquired in the United States.

Common signaling practices[edit]

Signal types[edit]

North American signals are commonly of three types.

• Absolute - Absolute signals are usually connected to an interlocking controlled by a block operator or train dispatcher. Their most restrictive aspect is "Stop" and trains cannot pass them at Stop unless they obtain special authority. Absolute signals will default to displaying Stop unless expressly cleared by a control authority. In older practice, multiple signal heads are directly above and below each other on the mast.

• Automatic - Automatic signals are governed by logic connected through electrical track circuits which detect the presence of trains or obstructions automatically. Automatic signals are permissive with their most restrictive aspect being one of the "Restricted Proceed" variety. Trains can pass an automatic signal displaying "Restricted Proceed" without any outside permission. Automatic signals are typically recognized by having an attached number plate and in older practice, having multiple signal heads offset from each other on the mast (i.e., on opposite sides of the mast).

• Semi-Automatic - Semi-Automatic signals are those that typically act as an automatic signal, but can be set to display an absolute "Stop" aspect. Semi-Automatic signals do not have a number plate, but can display an explicit "Restricted Proceed"-type signal.

Other types of signals can include Train Order signals, manual block signals or signals governing special safety appliances such as slide fences, non-interlocked sidings, road crossings, etc. These are much less common than the three standard types.

North American signals generally follow a common layout. A high signal consists of one to three heads mounted roughly in a vertical stack, each head capable of displaying one to four different aspects. Automatic signals are identified with a number plate whereas absolute signals are not. The signal's aspect is based on a combination of the aspects each individual head displays. Where a signal has multiple heads, aspects are read from top to bottom and are described as "X over Y over Z."

Dwarf signals are smaller signals used in low speed or restricted clearance areas. Most signaling aspect systems have a parallel set of aspects for use with dwarf signals that differ from aspects used in high signals. Dwarf signals may have multiple heads just like a high signal, but sometimes dwarf signals use so-called "virtual heads" to save on space and cost. This is where a dwarf signal displays multiple lamps on what would ordinarily be a single signal head creating the effect of multiple signal heads. For example, a stack of dwarf lamps in the order Yellow/Red/Green can display plain Yellow, Red and Green as well as Yellow over Green and Red over Green.

Behind the signal head is placed a dark backing or target, which helps improve signal visibility in bright ambient lighting. Target designs vary, but are usually round or oval, depending on the layout of the signal lamps. For each type of signal there are usually a range of target dimensions that can be chosen by the individual railroad company. As dwarf signals are not designed to be seen from long distances, they are not generally equipped with targets.

Signals are most commonly mounted on trackside masts about 12 feet (3.7 m) to 15 feet (4.6 m) high to put them in the eyeline of the engineer. Signals can also be mounted on signal bridges or cantilever masts spanning multiple tracks. Signal bridges and masts typically provide at least 20 feet (6.1 m) of clearance over the top of the rail. Bracket masts are arranged with multiple signals are mounted on the same masts governing two adjacent tracks. Bracket masts tend to be the tallest type of signal to allow the train crew to see the signal over a train on the intervening track. Signals in electrified territory may be mounted on the catenary structure, and signals on bi-directional lines may be mounted back-to-back on the same mounting device.

Through the 1970s signals were commonly mounted to the right of the track they governed. This mounting was designed to allow the engineer to view the signal when driving a steam or diesel locomotive with a long nose that restricted the view to the left. Where bi-directional running was implemented, signals needed to be mounted above the track or on bracket masts to allow this right hand placement. As locomotive design changed to allow good visibility on both sides of the track railroads shifted to bi-directional mast type signals, using signal bridges only in special situations involving multiple tracks or restricted views.

Dwarf signals are typically mounted on the ground in areas of low speed movements or restricted clearances. Dwarf signals may be sometimes mounted higher up on a small mast or other structure for improved visibility. These can be known as "high dwarfs" or "stick signals," but a tall mounting does not change the lower speed applications of the dwarf signal.

Signal colors and lamps[edit]

Electric signal lamps are typically low power (35 watt) incandescent lamps running off of low voltage DC current or, more recently, high intensity LED arrays. Incandescent signals use a doublet lens combination to directionally focus their small power out over a long range (3,500 feet in daylight.) New LED signals may either use an unfocused array or act as a drop-in replacement behind a traditional lens. U.S. signal lenses have a standard diameter of 8.375 inches (21.27 cm). North American signals use a standard set of colors, defined in October 1905, and which became common to other modes of transportation as shown on page 384 of the Simmons-Boardman 1911 Signal Dictionary.

• Green - Used to indicate "clear" or proceed.
• Yellow - Used to warn the engineer of an impending stop or speed reduction for an occupied "block" ahead. Also used for low-speed movements.
• Red - Used to indicate a full stop or other restrictive condition, or used as a "placeholder" light (when that part of a signal is unused but to confirm to the crew the signal is working, so as not to require guessing the rest of the combination in case of a light failiure).
• Blue - When on a signal doll arm, indicates intervening track between the signal and the track to which the signal applies, or to indicate all equipment on the section of track to the rear of the blue signal is absolutely not to be moved as men are working under, on, or in said equipment.
• Purple - Obsolete. No longer used on "Derails" and declared illegal in 1952 as a dwarf signal "Stop" indication color (pursuant to an Interstate Commerce Commission (ICC) ruling).
• Lunar White - Blue filtered light to eliminate all trace of yellow used to indicate a restricted proceed condition.
• Lemon-Yellow - Used in position light systems as an all-purpose high visibility color, greatest fog penetration.
• (Plain) White - Plain incandescent white light. Used in dwarf position light signals with frosted lenses.

Individual signal heads may be set to flash a color to create a different signal aspect. Signals in the United States typically flash only one head at a time, while signals in Canada may flash two heads at a time.^{[citation needed]}

Speeds[edit]

Signal rules and aspects make use of several pre-defined speeds. These speeds are also used in Weak Route type signaling.

• Normal Speed - The normal speed for the railroad line, also known as Maximum Authorized Speed (MAS).
• Limited Speed - A speed less than Normal Speed that was employed starting in the 1940s for use with higher speed turnouts (switches). This speed is defined by individual railroads and ranges anywhere from 40 miles per hour (64 km/h) to 60 miles per hour (97 km/h).
• Medium Speed - Original concept for a standard "reduced" speed normally set to 30 miles per hour (48 km/h) and can range as high as 40 miles per hour (64 km/h). This is the typical speed for diverging movements through interlockings and is also the speed trains are limited to when approaching a Stop or Restricted Proceed-type signals.
• Slow Speed - 15 miles per hour (24 km/h) while within the limits of an interlocking and 20 mph when not in the limits of an interlocking. This is used for trains negotiating complex trackwork at interlockings.
• Restricted Speed - Used for trains entering or operating in unsignaled territory or when entering a de-energized track circuit. Regulatory definition of no greater than 20 miles per hour (32 km/h) outside interlocking limits, 15 mph within interlocking limits. Trains operating at restricted speed must be able to stop within half vision short of any obstruction, and must look out for broken rails.

Fault tolerance[edit]

Signal aspects are designed to incorporate some degree of fault tolerance. Aspects are often designed so that a faulty or obscured lamp will cause the resulting aspect to be more restrictive than the intended one. Operating rules (GCOR, NORAC or CROR) require that dark or obscured signal heads be treated as displaying their most restrictive aspect (i.e. red), but fault-tolerant aspect design can help the engineer take a safer course of action before the failure of a signal becomes apparent. While not all aspects are fault-tolerant, the green lamp on the topmost head is only used by the least restrictive signal aspect, "Clear," so there is no case where a failure could accidentally display a clear signal.

Where a signal aspect incorporates a flashing lamp, the flashing lamp is always applied to less restrictive signals. This is to prevent a stuck flashing relay from accidentally upgrading the signal.

Some signaling logic incorporates "bulb out" (lamp failure) or other fault detection, to attempt to display the most restrictive aspect in case of a fault. However, this feature is not required nor universally adopted.

Signal types[edit]

Main article: Railway signal

Semaphore signals[edit]

Main article: Railway semaphore signal

Semaphore signals were first developed in England in 1841.^[3]:169 Some U.S. railroads began to install them in the early 1860s, and semaphores gradually displaced other types of signals. The Union Switch & Signal company (US&S) introduced an electro-pneumatic design in 1881. This was more reliable than earlier, purely mechanical versions, and more railroads began to use them. At that time, however, they were considerably more expensive than Hall disc, or "banjo", signals.^[3]:171

By the end of the 19th century, particularly as trains became longer and faster, and railroad lines grew more congested, the banjo signal was considered to have a single and terminal flaw: visibility. The internal disc was difficult to see in foggy weather and at night.^[4]:39 Earlier types of electro-pneumatic semaphores made by US&S had seen some limited application by 1880 as automatic block signals. The need to maintain air pressure in the long pneumatic lines eventually led the railroads to discontinue their widespread use as automatic block signals. However, these types did see long service in interlocking plants. Early semaphores also had limited range with manual wire operation and poor reliability in bad weather.^{[3]:149, 170-171} Thus some railroads continued to use disc signals where automatic block signal operation was needed between manual block stations as born out by period rule books well into the 1920s and beyond.

By the early 1890s more railroads began installing electric motor-operated semaphore signals, which were visible at distances of thousands of feet, during the day and under inclement weather conditions. In 1893 the high voltage, electric motor automatic block signal semaphore made its debut. By 1898, the US&S Style "B" semaphore, the first successful low voltage, entirely enclosed mechanism electric motor semaphore appeared. It was revolutionary, improving on all earlier semaphore designs, with the last such example being taken out of service as recently as 2009 on the former Siskiyou line of the S.P., now CORPS.

The motor-controlled North American semaphores used since the advent of the track circuit block system of 1872, provided a form of automation sought after by the railroads to reduce labor costs and improve reliability over manually operated systems as in the U.K., Germany and elsewhere. Dwarf signals were worked mechanically, pneumatically to give restricting-type signals as did mast type signals at interlockings, but motorized dwarfs were more common after the development of the Model 2A signal in 1908. As early as 1915, the technological push by -such intellectual giants as A.H. Rudd of the Pennsylvania R.R. and his concept of speed signalling combined with his development of the Position Light signal and the concurrent color-light signals using William Churchill's doublet lens combination in practical terms made the semaphore technically obsolete.

Semaphore signals have been almost completely replaced by light signals in North America, but they contain several important design elements. The overwhelming majority of semaphore type signals used in North America, and the only type surviving in service as of 2009 are of the three position, upper quadrant variety. Those of the lower quadrant variety would most often have two positions, but three lenses, two being of the more restrictive type. This was to reduce the chance of a malfunction or snowfall causing the signal to drop to its less restrictive position. Color images of these signals bear this out as the "Red-Red-Green" of the home and "Yellow-Yellow-Green" of the distant arms were universal on 60 and 70 degree (B&M, Central Vermont) L.Q. signals.

The board or "paddle" portion of the semaphore can take several shapes, each conveying a different meaning: - Those with a square end are "absolute" signals and generally force trains to stop when in their most restrictive position. - Those with a pointed end are "permissive" signals and permit a train to continue at lesser speed rather than having to stop completely. - Semaphores with a "fishtail" end (that is, a V-notch end) are "distant" signals conveying to the engineer what the aspect of the next signal is (as a forewarning). The color of the semaphore frequently matches the above categories as well, with absolute signals typically having a white stripe on a red board and the others having a black stripe (echoing the shape of the end) on a yellow board.

As of July, 2014, about two dozen active semaphores exist on a few segments of the former AT&SF now BNSF Railway's line through Glorietta Pass, Las Vegas and up through Wagon Mound in New Mexico.

Searchlight signals[edit]

In 1914 at Corning Glass's research facility in Corning, New York, the invention of the doublet lens combination for daytime color light signals prompted the management of the Hall Signal Company to realize that even their most advanced Style "L" semaphore mechanism (the very last produced by any U.S. signal company), had been rendered obsolete. That dual lens device had been developed by Cornell University's Dr. William Churchill, while he was working at Corning Glass Works. He had recently finished developing color standards for railroad glassware, which Corning had patented on October 10, 1905. The doublet lens combination was fully patented by 1911.

Hall's response to this (for them) dire situation was to buy the 1918 filed patents from one Mr. Blake for his "Searchlight" signal. In reality, the searchlight signal was an updated and modernized variation of the old Hall enclosed disc signal. What Blake had done was to harness the standard railroad three position polarized vane relay, add a miniature spectacle and Pyrex, low expansion Borosilicate glass roundels, and couple that with a very efficient elliptical reflector and optical lens system. This revolutionary development, provided a signal with a visible indication of over a mile from the signal in broad daylight, when the signal was located on tangent track. The early color light signals were visible for only about half that distance (2,500 feet) while using about the same current consumption, then a major concern in "Primary Battery Territory." By 1925, the development of "High Transmission Colors" of railroad glassware by Churchill and Corning Glass improved this limited distance to an acceptably competitive 3,500 feet on tangent track.

When the new Hall Searchlight signals were introduced in 1920, the recorded response by many engineman was classic: "They took the old 'Hall Banjo Signal,' resurrected it from the grave and lighted it up!"^{[citation needed]} In the U.K., original electromechanical searchlight signals consisted of a low wattage incandescent bulb mounted behind a semaphore spectacle devoid of a blade behind a target.^{[citation needed]}

Searchlight signal's use became widespread mostly due to their relatively low maintenance, high visibility, low power-consumption, and after 1932 using a compound lens with a 4 watt, 3 volt bulb, that worked quite well in territory with battery powered signaling. Also of significance was the single lens giving the indications in multiple head interlocking signals in a fixed location with regard to the mast and the other signal heads, this not being the case with multiple lenses color light signals. In time the costs of the significantly more expensive searchlight signal's relay began to outweigh the savings from its compact size and single bulb when compared with the simple multiple lensed color light signal. By the end of the 1980s the searchlight had lost its position as the most popular signal style in North America.^{[citation needed]}

To overcome the issues of associated with moving parts new solid state, single-lensed signals have been developed. The first such product, marketed in 1968 as the "Unilens" bySafetran Systems, uses fiber optics to concentrate the output of up to four light sources behind a single lens. However, other than as low speed signals requiring only short range visibility, these have not been entirely successful and most are now being removed from mainline service after a relatively short worklife.^{[citation needed]} Capable of four aspects, most examples had two lamp units simultaneously light red to give the most restrictive indication greater visual range than obtained with the use of a single lamp unit.

The latest single lens, multiple aspect signal technology involves the use of multiple colors of Light Emitting Diodes mounted together on a common plane to produce multiple colors from a single aperture. While now the standard colour light signal in the Britain, these have not been widely adopted outside of the U.K.^{[citation needed]}

Searchlight signals are typically mounted with a large circular background, with one or two railroads preferring a small target, such as the New York Central beginning in the mid late 1950s under the Pearlman administration.^{[citation needed]}

Triangular color light signals[edit]

Triangularly arranged color light signals consist of a cluster of three color lamp sockets in the middle of a large circular target. They were one of the first widely used type of high intensity color light signal, notably adopted by the New York Central and Seaboard Coast Linerailroads, and later used exclusively by Conrail and New Jersey Transit.^[5]

The original General Railway Signal (GRS) Type "G" design consisted of a cast iron box containing three doublet lens units in a triangular arrangement. The US&S "TR" and "TP" models used three smaller connected single lamp housings with a common background. The long defunct Chicago Signal Company had a version that used standard 5-3/8" switch lamp lenses (often of Macbeth manufacture) instead of the otherwise standard inverse-convex and stepped lens type found in the standard inner doublet design. The Union version was later updated to a single unit akin to the GRS model. As modular color light signals have become widespread, target-type configurations have been typically offered alongside vertical type configurations. The triangular color light signal was especially useful in physically restricted and confined areas.^[5]

Vertical color light signals[edit]

Vertical color light signals are the second major pattern of color light signals, and today represent the most popular form of signal in North America, supplanting the searchlight.^{[citation needed]} These signals are not different from the triangular type color signal in function, but present a much altered visual appearance.^{[citation needed]}

Continuing problems with reliable, long range light sources from a single, optical colored lens and a focused bulb restricted the first use of color light signals to short range daytime exterior applications, or tunnels and other underground or low speed complexes. The 1911 New York Penn Station project was one example of this type of color light signal, with an outer colored 8 3/8" optical lens, some of which are still in service as of 2011.^{[citation needed]}

Development of the doublet lens by Churchill at Corning Glass Works allowed an electric light source to be more effectively than with previous daytime colorlight signal designs. There are two main types of cases: the single case, where two or more lamps were contained within a single cast housing, and the modular light, where each lamp was an independent unit capable of being arranged into a signal of arbitrary configuartion, including triangular. US&S has a popular single case type with its styles R/R-2, P-2/5 and N, while GRS offered their triangularly arranged Type G, with the Chicago Signal Company providing a similar version. Today's Safetrans Triangular is a copy of the GRS Type G but with vertically arranged double doors.^{[citation needed]}

Signals like the model N/N-2 could also be mounted directly on the ground as a dwarf signal without a backing. The most notable user of this type of signal was the Chesapeake and Ohio, but units could be found on railroads all over the country.^{[citation needed]}

Over time, due to its low cost and versatility, the modular color light signal became the standard in North America. The first modular system was the GRS Type "D", first marketed in 1922, and adopted by the Southern Railroad along with many others: D&RG, etc. The GRS units used a smaller "background" than the comparable US&S vertical possibly somewhat compromising long range visibility. Today the most popular type of new signal in North America is a modular design manufactured by Safetran, as it is the cheapest, with all of the four major Class 1 railroads installing it almost exclusively.^{[citation needed]} Today, both GRS and Safetran market separate modular systems for high and dwarf signals, while US&S uses the single modular Style "R-2" design for high and Style N-2 for dwarfs.^{[citation needed]}

Modular color lights allow for all the cost savings inherent in color lights, but also make it easier for railroads to stock signals and perform alterations to interlockings. Instead of having to order custom heads, new modules can be taken from stock to build new signals or modify existing heads.^{[citation needed]}

With simple bracketry, even triangular colorlights may be built up with these standardized components.^{[citation needed]}

Another ubiquitous feature of modern modular color light signals is the full-length sun shade which improves visibility in bright sunny conditions. This shade was first developed by the Union Pacific to prevent snow buildup on one shade from obscuring the signal lens above it. Due to the appearance of the shade, signals of this type have been given the nickname Darth Vader by rail enthusiasts.^{[citation needed]}

Position light signals[edit]

Position light signals use rows of 5.375 inches (13.65 cm) diameter lamps to simulate the positions of an upper quadrant semaphore blade. Position lights were developed by A.H. Rudd, Superintendent of Signalling of the Pennsylvania Railroad (PRR). They were introduced in 1915 as a replacement for semaphore signals on the Main Line between Paoli and Philadelphia due to visibility problems caused by the new overhead electrification project. The original system used rows of four lights. The system was later reduced to use rows of three lamps, surrounding a common center. This reduced the "sail" effect of the inordinately large and tombstone shaped background of the four-light variant. The original installation made use of lamps in front of a free standing black sheet-iron backing, but after 1921 the new circular background (diameter 52 inches) was fitted to a 3 lamp per row device and directly to the backing on a framework referred to as a "spider."^[6]

Each position lamp unit has a 12 volt, 6 candlepower bulb in front of a parabolic mirror that increases the weak bulb's intensity. To avoid phantom indications the design uses an inverted toric lens (i.e. a single clear Fresnel lens mounted step sides outwards) with a portion of the lens steps painted black. A light yellow tinted conical glass with frosted tip was chosen, as this color was determined to have the highest visibility under fog conditions based on studies at Corning at that time.^[7]

A standard high position light consists of two heads; the bottom head can remain dark unless it is needed. In addition to the high position light signals the PRR developed a dwarf position light, as with many railroads, these dwarf signals are also referred to as a "pot," a tradition carried over from the 19th century revolving "Pot Type Signal." Four plain white lamps are able to display four low-speed aspects each with two lamps. In 1930, close clearances of the Philadelphia Suburban Station complex spurred development of the pedestal-type position, which consisted of two position dwarf signals in a common cast backing.^[7]

PRR type position lights were used throughout the vast PRR system as well as the Long Island Rail Road (LIRR), a PRR subsidiary, and the Norfolk and Western, which was one-third-owned by the PRR. US&S was the sole supplier of classic position light equipment; its factory was on the four track mainline of the P.R.R. in Swissvale, Pa.^[7]

In 1954 the PRR experimented installing two red lenses in the horizontal position of the upper head to help increase the at distance visibility of absolute Stop signals at Overbrook interlocking.^[8] Under the Penn Central and later Conrail it became standard practice to add these red lenses to high position lights and even some pedestal signals. The Norfolk and Western modified its signals to use red and green lenses in the upper head Stop and Clear positions and yellow lenses everywhere else. In the 1990s Amtrak modified most of its former-PRR position lights to use the equivalent color light colors in all of the positions of both heads. Internally referred to as position color lights, these are not be confused with color position lights described below, which while functionally similar are structurally considerably different.^[7]

New PRR type position lights continued to be installed up until the 1980s on former Conrail systems. Today most of the old PRR position lights are slowly being replaced by modern color lights, but Amtrak, SEPTA and the LIRR continue to install new position lights (Amtrak's being of the colorized variety). US&S no longer manufactures position light equipment, but updated models from Safetran are still available.^[7]

Color position light[edit]

The color position light (CPL) signal was developed by Frank Patenal, superintendent of signaling of the Baltimore and Ohio (B&O) railroad, in 1920. He also developed a proprietary signal aspect system to replace the earlier A.H. Rudd, ARA standard signaling system (PRR-based) then in use. The CPL system was unique in that it was a conceptually original design instead of being an update of an existing system. The CPL system incorporates several design principles that are otherwise unique to North American signaling. Use of the color red only in the case of an absolute stop or restricted speed situation is the most significant characteristic. The other 11 standard possible combinations do not display a red aspect.^[9]

The CPL consists of a central position target with up to four pairs of doublet lens units around the perimeter of the background disc. The lens units are spaced at 45-degree axes using the positions: green |, yellow /, red—and a lunar white \ for restricting also being present in some installations. The main head is surrounded by up to 6 markers at the 12:00, 2:30, 4:30, 6:00, 8:30 and 10:30 o'clock positions. The function of the main head was block occupancy information with green representing two or more clear blocks, yellow one clear block and red/lunar white representing a restricting indication, meaning the engineman was permitted to enter his train into an occupied block. The orbitals provide speed information, 12 o'clock being Normal speed, 6 being Medium speed (Limited speed if flashing), 10 being Normal to Medium (Limited if flashing), 2 being Normal to Slow, 8 being Medium to Medium, 4 being Medium to Slow and no lit orbitals being Slow to Slow.^[9]

This CPL was first deployed on the Staten Island Railroad (a B&O subsidiary) in the 1920s, and deployed system-wide shortly thereafter. Parts of the Chicago and Alton Railroadreceived CPLs later, when the B&O gained control of that line. In the 1980s both Amtrak's Chicago Union Station and Metra's Chicago Northwestern Station installed dwarf CPLs to replace earlier signals in those terminals.^[9]

As of 2008 and as with all U.S. Railroads, CSX is slowly replacing all of the remaining CPLs on its system with contemporary vertical color light LED signals. The signals on the oldAlton Railroad have also been almost entirely replaced as have many of the CPL dwarfs at the two Chicago terminals. The sole exception is the Staten Island Railroad, which recently upgraded its signaling system with new CPLs using modern Safetran position light equipment.^[9]

Obsolete mechanical and electrical signals[edit]

Early mechanical signals[edit]

The first signals employed on an American railroad were a system of flags used on the Newcastle and Frenchtown Turnpike and Rail Road in the 1830s. The railroad then developed a more effective system consisting of wooden balls, painted red, white or black, and hoisted up or down a pole on a rope-and-pulley system. The initial use of these signals was merely to indicate the on-time status of trains, rather than to control train movements. The wooden balls were often configured with lanterns for nighttime use.^[4]:18 Ball signals were first used to direct train movements in 1852, on the New York and New Haven Railroad.^[3]:134 Other mechanical signals used during the 19th century include:

• a 4 feet (1.2 m) disc, painted red and mounted on a revolving pole; it indicated "stop" when positioned to face an approaching train
• a pivoted board, called a "smashboard," which could be operated to swing into position across the track.^[3]:136

Hall disc signals[edit]

The Hall disc signal (a/k/a banjo signal) was the first electrically-operated signal to be widely adopted by American railroads. Thomas Hall patented his disc signal design in 1867.^[3]:146-147

A banjo-shaped wooden case housed a large iron wire hoop with red silk stretched and glued over it. The opposite end had a much smaller hoop in which a very thin disc of colored glass was secured. This entire iron wire assembly was pivoted inside an electromagnet on what was known as a "Z" armature which was wound with copper magnet wire. When the coil was energized, the wire hoops were moved away from the large glass opening in the front of the wooden "banjo" case exposing its white painted insides. The colored glass disc at the same time moving away from a clear primitive Fresnel lens at the top of the case which was backed up on the rear side of the case with a kerosene lamp.^[10]:271 The disc signal was first placed into service in 1870 on the New York and New Haven Railroad at Stamford, Connecticut, using a track treadle device to activate it, as the revolutionary track circuit was not developed until 1872 by Dr. William Robinson.^[11][12]

The Hall Signal Company installed the disc signals as part of automatic block signal systems, initially utilizing line wire circuits, running on poles alongside the tracks, connecting the track treadle devices. One of the earliest such systems was installed in 1871 on the Eastern Railroad (later theBoston & Maine).^[13]:18 About 1500 disc signals were operational by 1896.^[14]:80

The all metal Union Switch & Signal Enclosed Disc Signal was introduced in 1896 and had one version that employed both a red and a green banner (as well as both colored glasses) that were mechanically arranged in such a way as to have the banners and glass roundels exchange places within the signal case as the indication required.

Extant mechanical and electrical signals[edit]

There are examples of various mechanical and electrical signals in several railway museums and in the collections of a very few railroad enthusiasts. These include signals that were manufactured by US&S, GRS, Hall and even the Federal Signal Company. The Hall Company's 1921 variant of the dwarf color position light signal are amongst the rarest and most sought after, as are the extremely rare mechanical dwarf semaphores of the T.George Stiles Company used into the 1980s and beyond installed at the turn of the 20th Century by the former New Haven.

Common signal rule classes[edit]

Most North American railroads have between 10 to 20 separate signal rules, each which are often represented by multiple aspects. However, all of these complicated rules revolve around the simple premise of informing the locomotive engineers how they are to operate their train in the present location, and what they are to expect at the next signal location.^[15] From here the large set of rules and aspects can be broken down into a small number of classes which are common to all North American signaling systems.^{[citation needed]}

• Automatic Block - Block aspects convey basic track occupancy information and advise the engineer (operator) which of the basic signal rules (common to all railroads) he/she is to follow in the operation of his/her train at any point on the railway line. These include Clear, Advance Approach and Approach which instruct the engineer to "expect no stop", "expect stop at second signal" and "expect stop at next signal" respectively. Advance approach is only used in situation with short signal blocks to ensure trains have enough stopping distance. These are the most common signal aspects in North America and are the only aspects most automatic block signals need to display.

• Approach at Speed - When a train needs to be told to slow down due to dynamic conditions an "Approach Speed" aspect is used. These inform the engineer to slow to a prescribed speed by the next signal. The most common reason for this is that the train is to take a diverging, or non-Normal speed route at the next interlocking. Signals of this type include Approach Medium, Approach Limited, Approach Slow and Approach Diverging. These signals are typically displayed on the distant signal to an interlocking, but can sometimes be used with short signal blocks in place of Advance Approach.

• Diverge to Clear - This class appears only on absolute signals and informs the engineer that the train will be taking a diverging route and need not expect a stop at the next signal. In speed signaling the engineer is informed of the speed the train needs to take the route at, in weak route signaling the engineer is just informed of a diverging route. Signals in this class include Medium Clear, Slow Clear, Limited Clear and Diverging Clear.

• Diverge to Stop - Same as above only the train can expect to stop at the signal after the interlocking. These signals include Medium Approach, Slow Approach and Diverging Approach.

• Combination Signals - These combine functions of a "Diverge to" signal with an "Approach Speed" signal and occur in areas of complex trackwork where there are no intermediate signals between one interlocking and the next. In the United States only a few combination signals like Medium Approach Medium, Medium Approach Slow and Diverging Approach Medium/Slow are ever found in rulebooks and not frequently used in practice. The Canadian standard rulebook contains signal rules and aspects for every possible combination.^[16]

• Restricted Speed Signal - This class of signals is displayed for trains moving into a block where a track circuit has been de-energized or does not exist. A "shunted" track circuit indicates either the block is occupied by another train or railcar, or there is a problem such as a broken rail or flooded track. Where a track is not protected by track circuits that track must be presumed to be occupied. As the name implies this signal requires trains to move at Restricted speed, specifically with the ability to stop short of an obstruction. Restricted speed signals take many forms including Restricted and Restricted Proceed where trains must simply pass the signal at restricted speed and also Stop and Proceed, where a train must come to a complete stop before proceeding at restricted speed. Stop and Proceed has fallen out of favor with most freight railroads due to the fuel and time savings of allowing the trains to not come to a complete stop. This aspect class can be displayed on almost all railroads in North America.

• Stop Signal - Stop signals are displayed on Absolute signals, in fact the ability to display an absolute Stop is part of that signal type's definition. Stop is the most important signal as passing a signal at Stop presents a serious risk of accident. Engineers committing a Stop signal violation automatically have their Federal certification suspended and are frequently fired. Stop signals can only be passed upon special permission from a control authority.

• Cab Signaling Signals - Where cab signaling is employed without fixed trackside automatic signals, special signal aspects are required at absolute signals. These include some sort of absolute block "Super Clear" signal that allows passage to the next interlocking with a fixed signal and also the "Cab Speed" signal that informs the engineer to proceed under direction of cab signals.

Distant (approach) signals[edit]

A distant signal can either be an automatic signal before an interlocking, or the interlocking signal itself when interlockings are back to back. Distant signals typically display more aspects than a typical block or interlocking signal to warn trains of diverging movements at the next interlocking however this is not always the case if there are no diverging paths available.^{[citation needed]}

Distant signals are often referred to as Approach Signals as the signal block before the interlocking is known as the approach block. When a train enters the approach block any route lined up at the interlocking will become locked in place until a timer is run to prevent routing a conflicting movement without giving the approaching train adequate time to come to a stop.^{[citation needed]}

In the aftermath of the 1996 Silver Spring Collision, the Federal Railroad Administration amended its regulations for push-pull trainoperation to prevent locomotive engineers from forgetting that they were approaching a stop signal after making a station stop. The resulting "Delay in Block Rule" requires that all distant signals, located in territory where push-pull trains operate in the absence of cab signals, be marked with a "D" placard. The placard is intended to remind engineers that they are bound by a 40 miles per hour (64 km/h) speed restriction, and must approach the interlocking signal prepared to stop, whenever a station stop is made or train speed drops below 10 miles per hour (16 km/h) in the approach block. The restrictions hold until the interlocking signal is clearly visible and is displaying a "proceed" indication.^[17]