Cab signalling: Wikis


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The cab signals in a Chicago 'L' train. The vertical light bar in the middle of the signal indicates the maximum permitted speed for the section of track where the train is currently located.

Cab signalling is a railroad safety system that communicates track status information to the train cab (driving position), where the engineer or driver can see the information. The simplest systems display the trackside signal aspect (typically, green, yellow or red, indicating whether it is safe to proceed or not), while more sophisticated systems also display allowable speed, location of nearby trains, and dynamic information about the track ahead. In modern systems, a train protection system is usually overlaid on top of the cab signalling system to warn the driver of dangerous conditions, and to automatically apply the brakes and bring the train to a stop if the driver ignores the dangerous condition.[1] These systems range from simple coded track circuits to transponders that communicate with the cab, to communication-based train control systems.



The main purpose of a signal system is to enforce a safe separation between trains and to enforce speed limits. The cab signal system is an improvement over the wayside signal system, where visual signals beside or above the right-of-way govern the movement of trains, without any means of enforcing the signal automatically. While early cab signal systems only repeat the wayside signal aspect displayed, all modern systems have an enforcement component which can automatically bring a train to a stop.

The first such systems were installed on an experimental basis in the 1910s in the United Kingdom, 1920s in the United States, and later in the Netherlands in the 1940s. High-speed trains such as those in Japan, Northeastern United States, Britain, France, and Germany use such systems in principle, though they are mutually incompatible in practice.

The European Rail Traffic Management System (ERTMS) is a multi-national standard that is progressively being developed in Europe, with an aim to improve interoperability. The train-control component of ERTMS, termed European Train Control System (ETCS), is a functional specification that incorporates the former national standards of several European countries. The German Indusi, German LZB, British TPWS, and the French TVM could all be made ETCS-compliant with modifications.

In North America, the power-frequency coded track circuit system developed by the Pennsylvania Railroad and Union Switch & Signal (US&S) is the de-facto standard in the Northeast. Variations of this system are also in use on many rapid transit systems, including the MBTA Red Line, London Underground's Victoria Line, and form the basis for the first generation Shinkansen signalling developed by Japan National Railways (JNR).

Hierarchy of cab signal systems

With a traditional signalling system, the engineer/driver must observe wayside signals and act accordingly, depending on the aspects displayed. This method of operation is susceptible to human failure; if the engineer does not respond to a signal aspect, a dangerous situation may result. It is also considered 'passive', in that the system does not actively prevent the unsafe conditions from arising. Modern cab signalling is an active system, in that the train will default to a safe condition (brakes applied) if left unattended as it approaches an adverse signal aspect. Thus, the reliance on humans to ensure absolute safety is somewhat reduced.


Intermittent, Continuous, and PTC

There are three classes of cab signal systems:

  • Wayside signals with intermittent warning and enforcement capabilities
  • Continuous cab signal and speed enforcement systems
  • Positive Train Control (PTC) and other new technology systems

Typically, when the term 'cab signal system' is used, it refers to the continuous cab signal and speed enforcement system. However, the intermittent systems are also considered a form of cab signalling.


Metro North Cab Sigal.jpg

These systems provide for the transmission of information about approaching signal aspects to the train and some level of enforcement of those aspects. The most basic level of enforcement are the simple Automatic Train Stop (ATS) systems that apply the brakes if a train passes a stop signal. Such systems have been widely used on heavy rail transit systems since the earliest days. On main line railroads, initiating braking at a stop system would be ineffective because of higher speeds and lower braking rates. Instead a warning is transmitted to the train on passing an approach aspect requiring a speed reduction. If the warning is not acknowledged after a preset time (usually 8 seconds), brakes are applied. If the warning is acknowledged a warning indication is displayed in the cab to the next clear signal, but the engineer is fully responsible for operating the train.

These systems have the obvious disadvantage that an engineer might acknowledge the warning but fail to take action to slow the train. To reduce this risk, newer types of ATS have been developed, such as PZB 90 in Germany and Train Protection & Warning System (TPWS) in the UK to enforce a speed reduction before the stop signal, and fully apply brakes if the signal is passed. The newest systems (such as ASES on New Jersey Transit commuter lines) use line side transponders to transmit complex messages to the passing train. Collision risk is reduced, but not completely eliminated. These systems are a compromise between intermittent and more costly systems with continuous track-train communications that provide almost complete assurance against human error collisions.


These systems (generally known as Automatic Train Control (ATC) systems) use the rails or loop conductors laid along the track to provide continuous communication between wayside signal systems and the train.[2] The most widely used systems in the United States use coded track circuits to transmit and display the aspect of the approaching signal in the cab. An on-board speed enforcement system ensures that the speed for that signal aspect is observed when passing the signal. However, in their traditional form, the systems cannot enforce an absolute stop at a stop signal, since they do not have a way of determining precise train location and distance to the next signal. The more advanced ATC systems such as the LZB in Germany and the TVM series in France do have this capability. LZB and TVM systems are applied primarily on new high speed passenger lines. In the United States, the addition of the ACSES transponder-based speed enforcement systems provides the absolute stop, as well as enforcement of civil speed limits on portions of the Northeast Corridor.

PTC (Positive Train Control)

Positive Train Control is the term given in North America to a family of train control technologies that provide for an enforced stop at absolute stop signals and preliminary speed control to make an enforced stop possible. While already implemented with passenger trains, long freight trains present a harder problem due to non-uniform handling characteristics.

Existing PTC systems are mostly transponder based and overlay with the existing signaling system by transmitting state and distance information to an on board computer that calculates a safe braking profile.

PTC systems being developed on normally un-signaled lines or to reduce the cost overhead of traditional signaling systems use wireless data communications between the control center and trains. Such systems are known as Communications Based Train Control (CBTC). CBTC comprises a variety of systems used in lieu of traditional block signals and interlockings for managing safety and capacity, especially non-traditional ways of determining exact train location, for example using GPS and inertial navigation, and digital radio for communications between trains, wayside devices (such as switches and grade crossings) and zone controllers and the control center.

These systems are being trialed on urban heavy rail rapid transit systems (such as the pilot CBTC installation on the L line (Canarsie) of the New York City Subway System), and are being widely tested in main line applications. Most of these systems are not fully mature, and are still in development or service trials and face numerous problems maintaining a constant bi-directional wireless link between vehicles.

Information transmission

Cab signals require a means of transmitting information from wayside to train. There are a few main methods to accomplish this information transfer:

  • Mechanical
  • Magnetic field
  • Electric current
  • Inductive (changing magnetic field)
  • Coded track circuits


The most basic type of cab signal system is one that relies on mechanical contact between wayside equipment and the train. The New York City Transit Authority (NYCT), and the London Underground uses this system to this date on some of its lines.[2] This is known as the 'trip-stop' system (also called Train stop). If a stop signal is displayed, a trip-arm located next to the running rails becomes 'raised'. On the bogies/trucks of the underground/subway cars, there is an emergency brake valve. If a train would overrun a stop signal, the raised arm would strike the emergency brake valve, causing the train's compressed air line to depressurize or be 'dumped'.[3] The brakes throughout the entire train are automatically applied.

This form of Automatic Train Stop (ATS) is considered a cab signal system, albeit a rudimentary one. It provides exactly two aspects, "stop" and "go". It does nothing to regulate speeds, does not display the signal aspect inside the cab, and does not enforce a signal aspect until a stop signal overrun actually occurs. However, it is much safer than not having a cab signal system at all.

Magnetic field

A variation of the mechanical contact system is to use the absence of a magnetic field to designate a hazardous condition.[4] The British Rail AWS (Automatic Warning System) is an example of an automatic train stop system transmitting information using a magnetic field.

This form of automatic warning system is also a two-aspect cab signal system. It does not provide absolute stop enforcement although its systemwide installation has prevented many accidents on Britain's railways. Because of a series of high profile SPAD (Signal passed at danger) incidents, from around 1999 AWS was supplemented by the Train Protection & Warning System (TPWS), which provides stop signal enforcement.

Electric current

The magnetic systems are non-contact and are generally preferred -- contact between a fast moving train and wayside equipment leads to a lot of wear and tear. However, in the early part of the 20th Century, the Great Western Railway in Great Britain experimented with an electric system, whereby long bars in the 'four-foot' between the rails became energized with an electric current (supplied from a battery) when the distant signal was clear. This system is described in the article on Automatic Warning System.


Inductive system are non-contact systems that rely on more than the simple presence or absence of a magnetic field to transmit a message. Inductive systems typically require a beacon or an inductive loop to be installed at every signal and other intermediate locations. The inductive coil uses a changing magnetic field to transmit messages to the train. Typically, the frequency of pulses in the inductive coil are assigned different meanings.

Examples of inductive systems include the German Indusi system, and the British TPWS.

Coded track circuits

A coded track circuit based system is essentially an inductive system that uses the running rails as information transmitter. The coded track circuits serve a dual purpose: to perform the train detection and rail continuity detection functions of a standard track circuit, and to continuously transmit signal indications to the train. In so doing, the coded track circuit systems also eliminate the needs for specialized beacons.

Examples of coded track circuit systems include the Pennsylvania Railroad standard system, a variation thereof used on the London Underground's Victoria Line,[5] and one used on the MBTA Red Line. Newer audio frequency (AF) track circuit systems are used on the Hudson-Bergen Light Rail (HBLR) and Newark Light Rail. The AF track circuits differ from traditional power-frequency coded track circuits in that it relies more heavily on digital signal processing to transmit and detect information but can be cheaper and simpler to design and implement.

Typology of cab signal systems

Intermittent, single-aspect Intermittent, multi-aspect Continuous
Great Western Railway, UK (Reading-London 1910, all mainlines by 1930) Indusi I-60R (1960), I-90 (Alcatel 6641) Pennsylvania Railroad pulse code system, Lewistown test installation (1923); Main Lines (including Northeast Corridor) (1930s), speed enforcement capabilities added in 1950s
Trip-stops: New York City Transit, Systemwide (current); MBTA Red Line (1970s) General Railway Signals (GRS)'s automatic train stop (ATS) system deployed on Chicago & North Western (CNW), Aitchson Topeka & Santa Fe (ATSF), and New York Central (NYC) -- installed in the 1960s
British Rail AWS (Automatic Warning System) British Rail TPWS (Train Protection & Warning System) Audio-Frequency Track Circuit systems installed on Newark City Subway, Hudson-Bergen Light Rail
Indusi for Ottawa's O-Train, Early German Indusi PZB and I-54 (1954) Bulgaria Alcatel 6413 Pennsylvania Railroad/Long Island Rail Road's Automatic Speed Control (1953); Amtrak's Shore Line implementation in 1997 uses ACSES which has the PRR pulse codes at its core; Metro-North and New Jersey Transit Commuter Rail Operations both implemented similar systems.
Magnetic Train Stops on the NJ Transit River Line (New Jersey Transit) India Ansaldo Pilot London Underground Victoria Line (PRR derivative system)
SNCF's KVB classic line -- KVB = Contrôle de Vitesse par Balises (Beacon-based Speed Control) SNCF's TGV uses TVM-300 and TVM-430, which are track-circuit based. TVM = Transmission Voie-Machine (Track-Train Communication)
Chicago Transit Authority (CTA), Massachusetts Bay Transportation Authority (MBTA, 1980-1990s), WMATA's automatic train operation (ATO), PATCO's ATO (1969), BART's ATO (1975) all use some variation of coded track circuit systems for cab signalling and automatic train control with speed enforcement.
Florida East Coast Railway uses a cab signal system with speed enforcement on its main line

Cab signal systems in the US

Cab signaling in the United States was driven by a 1922 ruling by the Interstate Commerce Commission that required 49 railroads to install some form of automatic train control in one full passenger division by 1925.[6] Now while several large railroads including the Santa Fe and New York Central decided to fulfill the letter of the requirement by installing intermittent inductive train stop devices, the Pennsylvania Railroad saw an opportunity to improve operational efficiency and installed the first continuous cab signal systems, eventually settling on pulse code cab signaling technology supplied by Union Switch and Signal.

In response to the Pennsylvania Railroad lead, the ICC mandated that some of the nation's other large railroads had to equip at least one division with continuous cab signal technology as a test to compare technologies and operating practices. The affected railroads were somewhat less than enthusiastic and many chose to equip one of their more isolated or less trafficked routes to minimize the number of locomotives needing to be equipped with the cab signal apparatus.

Several railroads chose the inductive loop system rejected by the PRR. These included the Central Railroad of New Jersey (installed on its Southern Division), the Reading Railroad (installed on its Atlantic City main line) and the New York Central. Both the Chicago Northwestern and Illinois Central employed a two-aspect system on select suburban lines near Chicago. The cab signals would display "Clear" or "Restricting". The CNW went even further and also eliminated the wayside intermediate signals in the stretch of track between Elmhurst and West Chicago, requiring trains to proceed solely based on the 2 aspect cab signals.

As the Pennsylvania Railroad system was the only one adopted on a large scale, it eventually wound up becoming a de facto national standard and most installations of cab signals in the current era have been of this type. Recently there have been several new types of cab signaling which seek either to use communications based technology to reduce the cost of wayside equipment or supplement existing signal technologies to enforce speed restrictions, absolute stops and respond to grade crossing malfunctions or incursions.

The first of these was the Speed Enforcement System employed by New Jersey Transit on their low density Pascack Valley Line as a pilot program using a dedicated fleet of 13 GP40PH-2. SES used a system of transponder beacons attached to wayside block signals to enforce signal speed. SES was near universally disliked by engine crews due to its habit of causing immediate penalty brake applications without first sounding an overspeed alarm and giving the engineer a chance to slow down. SES is in the process of being removed from this line, and is being replaced with CSS.

Amtrak decided to adopt the Advanced Civil Speed Enforcement System or ACSES for its new high speed rail service to Boston. ACSES was an overlay to the existing PRR type CSS and used the same SES transponder technology to enforce permanent and temporary speed restrictions at curves and other geographic features on the line. The on board cab signal unit would process both the pulse code "signal speed" and the ACSES "civil speed" and then enforce the lower of the two. ACSES also provided for a positive stop at absolute signals which could be released by a code provided by the dispatcher transmitted from the stopped locomotive via a data radio. Later this was amended to a simpler "stop release" button on the cab signal display. ACSES is slowly being implemented along the Northeast Corridor between Boston and Washington, but in most cases it only is used by the high speed transits.

Positive train control

Positive train control systems can be overlaid onto cab signal systems or can replace them altogether. These include the Incremental Train Control System (ITCS) installed by GE on the Chicago-Detroit route in Michigan and the North American Joint Positive Train Control (NAJPTC) system being testing in Illinois on the Chicago-St. Louis route. The ITCS has been in revenue service since 2002 with speeds up to 90 mph (145 km/h).[7] Other systems include Alaska Railroad's CAS, CSX's CBTM, and BNSF's ETMS and V-ETMS manufactured and installed by Wabtec.[8]

Audio frequency track circuits

While pulse-coded track circuits have on/off cycles measured in pulses per minute, as their name implies, audio-frequency track circuits have signal frequencies in the range between 2000 Hz and 20000 Hz. Because of the relatively high frequency, the signal quickly attenuates, and while pulse codes can travel for several miles, audio frequency codes can only travel between a few hundred and a few thousand feet. However, this has an advantage in that by carefully matching the carrier frequency to the block length, the need for insulated rail joints can be largely eliminated. In rapid transit and light rail systems, where high traffic density mandates short signal blocks, the lack of a need for insulated rail joints (and impedance bonds) can result in significant cost savings.

Like standard track circuits, the audio frequency track-circuits provide positive train detection and like pulse code cab signals they provide the transmission of signal aspects which can change between block boundaries. Also like the pulse code system the vehicle borne equipment reads the code embedded in the audio frequency carrier and then passes this on to the train control system to alert the operator and/or reduce train speed as necessary.


  1. ^ Elements of Railway Signaling, General Railway Signal (June 1979)
  2. ^ a b "NTIS order #PB-254738 - Automatic Train Control in Rail Rapid Transit" (PDF). United States Congress Office of Technology Assessment. May 1976.  
  3. ^ "Subway Signals: A Complete Guide".  
  4. ^ Railway Signalling -- A guide to modern signalling technology, Institution of Railway Signal Engineers. Published 1980.
  5. ^ "Automatic Train Operation on the Victoria Line". Tubeprune. 2003-03-15. Retrieved 2008-03-13.  
  6. ^ Railroad Operational Safety, Status and Research Needs (Transportation Research Circular E-C085 ed.), Transportation Research Board of the National Academies, January 2006, p. 27,, retrieved 2008-04-13  
  7. ^ United States Federal Railroad Administration (2004-12-15). "Positive Train Control Overview". Retrieved 2007-01-10.  
  8. ^ BNSF promotional video.

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