Cab signalling

Cab signalling

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 speed enforcement system usually
overlays 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. 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 is also in use on many
rapid transit systems, including the MBTA Red Line, London Underground’s
Victoria Line, and forms 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/operator 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. Cab signal is an active system, in
that the train will default to a safe condition (brakes applied) if left
unattended as it approaches an averse 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.


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 present
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


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. 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.

(Positive Train Control)

Positive Train Control is the term given in
North America to a family of train control technologies that aims to provide the
functionality of the most advanced ATC systems described in the previous section,
plus some additional functions, at a much lower cost.

PTC systems typically rely on wireless data communications between the control
center and trains to minimize costly wayside installations. 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 increasingly common on urban heavy rail rapid transit systems
(such as the pilot CBTC installation on the Canarsie Line of the NYCT), 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.

Information transmittal

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. 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 become ‚dumped‘. The
brakes throughout the entire train is 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. 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

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 described below, 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, and can be cheaper and
simpler to design and implement.

Typology of cab signal systems

Intermittent, single-aspect

Intermittent, multi-aspect


Great Western Railway, UK (Reading-London
1910, all mainlines by 1930)

Indusi I-60R (1960), I-90 (Alcatel 6641)

Pennsylvania Railroad, Lewistown test installation (1923); Northeast
Corridor (1930s), with enforcement capabilities added in 1950s

Trip-stops: New York City Transit,
Systemwide (current); MBTA Red Line (1970s)


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

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

Underground Victoria Line (PRR derivative system)


SNCF’s KVB classic line — KVB = Contrôle
de Vitesse par Balises (Beacon-based Speed Control)

TGV uses TVM-300 and TVM-430, which are track-circuit based. TVM =
Transmission Voie-Machine (Track-Train Communication)



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



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.

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 isolates 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 more simple "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.

Pennsylvania Railroad Pulse Code Cab Signal

The first widely implemented railroad cab
signaling system was employed by the Pennsylvania Railroad in the 1920s and
continues to be the dominant cab signal system in North America. The Interstate
Commerce Commission ruled in 1922 that trains would be limited to 80mph without
some sort of automatic train stop system and the Pennsylvania Railroad decided
to address this ruling with a technology that could improve both safety and
operational efficiency. This came in the form of a signal displaying
continuously in the cab of the locomotive based on electrical current
transmitted through the rails. The technology was developed by the Union Switch
and Signal corporation, the PRR’s preferred signal supplier.

The first test installation between Sunbury, PA and Lewistown, PA consisted of
turning the tracks into an inductive loop with three aspects (RESTRICTING,
APPROACH and CLEAR) and signals transmitted using commercial 60Hz current. The
test installation also did away with wayside block signals with trains relying
solely on cab signals. The PRR installed another "loop" system on the Northern
Central line between Baltimore, MD and Harrisburg, PA in 1926, this time using
special 100Hz current.

In 1927 the PRR began to test another variation of Cab Signals which used pulse
codes instead of inductive loops. The pulses of 100Hz current were detected via
induction by a sensor hanging a few inches above the rail before the leading set
of wheels. The codes were 180 ppm for CLEAR, 120ppm for APPROACH MEDIUM, 75ppm
for APPROACH and 0 for RESTRICTING. The pulse rates were chosen to avoid any one
rate being a multiple of another leading to reflected harmonics causing false
indications. The system was failsafe in that the lack of code would display a
RESTRICTING signal. The codes would be transmitted to the train from the block
limit in front of it. This way if the rail was broken or another train entered
the block, any codes would not reach the onrushing train and the cab signal
would again display RESTRICTING.

US&S electro-mechanical pulse code generator unit generating 180ppm for a cab
signal system.

Initially the cab signaling system only acted as a form of automatic train stop
where the engineer would have to respond to any drop in the cab signal to a more
restrictive aspect to prevent the brakes from automatically applying. Later,
passenger engines were upgraded with speed control which enforced the rulebook
speed associated with each cab signal (APPROACH MEDIUM = 45mph, APPROACH =
30mph, RESTRICTING = 20mph).

Over time the PRR installed cab signals over much of its eastern system from
Pittsburgh to Philadelphia, New York to Washington. Where DC and AC
electrification co-exist the standard 100Hz frequency is changed to 91 2/3Hz to
avoid even harmonics created by the 25Hz traction current flowing through DC

This system was then inherited by Conrail and Amtrak and various commuter
agencies running on former PRR territory such as SEPTA and New Jersey Transit.
Because all trains running in cab signal territory had to be equipped with cab
signals, most locomotives of the aforementioned roads were fitted with cab
signal equipment. Due to the effect of interoperability lock in, the 4-aspect
PRR cab signal system has become a de-facto standard and almost all new cab
signaling installations have been of this type or a compatible type.

Current lines using the 4-aspect PRR Cab
Signal System

  • Amtrak Northeast Corridor Line
  • Amtrak Harrisburg Line
  • Amtrak Springfield Line
  • Amtrak Michigan Line
  • Amtrak Shore Line (no waysides New
    Haven to Providence)

  • Norfolk Southern Pittsburgh Line
  • Norfolk Southern Port Road Line
  • Norfolk Southern Conemaugh Line (no

  • Norfolk Southern Morrisville Line (no

  • Norfolk Southern Fort Wayne Line (Conway
    Yard to Alliance, OH, no waysides)

  • Norfolk Southern Cleveland Line (Alliance,
    OH to Cleveland, OH, no waysides)

  • CSX Boston Line (no waysides)
  • CSX Hudson Line
  • CSX Landover Line
  • CSX RF&P Sub (formerly using RF&P CSS
    system at 60 Hz)

  • CSX (RF&P) Sub (modified to 100 Hz-(PRR
    type) from original 60 Hz system)

  • NJT All Lines
  • Metro-North All "East of Hudson" Lines (no

  • MBTA Old Colony Lines (no waysides)
  • SEPTA Main Line
  • SEPTA Airport Line
  • SEPTA Chestnut Hill West Line
  • SEPTA Neshamity Line
  • SEPTA Fox Chase Line

Related Pulse Code Systems

  • Long Island Rail
    Automatic Speed Control:
    The LIRR was a PRR subsidiary and
    it is no surprise that they would adopt a similar system. The LIRR used
    standard PRR cab signals until bought by the Metropolitan Transportation
    Authority in 1968 when it was modified slightly into ACS systems used to
    this day. ASC employs two additional codes, 270 and 420 ppm and replaces the
    in-cab signal display with an in-cab speed display. Th additional codes are
    used to display speeds of 50 and 60 mph, which are used to slow trains for
    curves, higher speed turnouts and short signal blocks.

  • Chicago,
    Burlington and Quincy
    Aurora Line Cab Signals:
    The CB&Q commuter
    line to Aurora, Illinois used the same technology as the Pennsylvania, just
    with different rules and wayside indications to conform to their partly
    route-based signaling system. It remains in service to the present day.

  • Union Pacific
    Automatic Train Control:
    The Union Pacific has implemented the PRR type
    technology on much of its main line between Chicago and Wyoming, as well as
    several other lines on its system in recent years. As with the CB&Q cab
    signals, the system works the same as the PRR, but uses different rules and
    partly route based wayside signals.

  • METRA Rock Island
    Automatic Train Control:
    Another PRR based cab signal system left over
    from the Rock Island heyday. It can be found on the Joliet IL to Chicago
    portion of Metra, the Metra Rock Island District

  • Rapid Transit Lines: Various rapid
    transit lines built or re-signaled in or before the 1990s make use of the
    pulse-code cab signal technology for both manual or automatic train
    operation schemes. Rapid transit systems are typically failsafe with a O
    code mandating a complete stop of the rail vehicle. Also, the complete range
    of pulse codes are used to provide the maximum granularity in speed control.
    Some examples include the PATCO Speedline in Philadelphia, the SEPTA Route
    100, the Baltimore Metro and the Miami-Dade Metrorail. Pulse-code technology
    on rapid transit lines has generally been supplanted by Audio-Frequency cab

  • MTA Staten Island
    Automatic Speed Control:
    A hybrid of the PRR/LIRR systems
    and Rapid Transit power-frequency cab code. The ATC applies service braking
    in response to overspeed conditions. 75-120-180-270 are used as speed
    commands. Zero code is used for stop rather than restricting, which is
    50PPM. 420 is used as a latch-out. Dispatchers may authorize trains stopped
    by a zero-code to close in on certain interlocking signals by manually
    activating a 50PPM close-in code.

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). Other
systems include Alaska Railroad’s CAS, CSX’s CBTM, and BNSF’s ETMS.

Audio frequency track circuits

While pulse-coded track circuit have on/off
cycles measured in pulses per minute, as its name implies, audio-frequency track
circuit 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 =
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.
2 =  
Modern Amtrak cab signal display using colorized position light signals.
This unit is compatible with the ACSES overlay system and is currently
displaying a RESTRICTING aspect with the speed limit of 20mph displayed
3 =  
US&S electro-mechanical pulse code generator unit generating 180ppm for
a cab signal system.

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