Cab signalling
Encyclopedia
Cab (locomotive)
The cab, crew compartment or driver's compartment of a locomotive is the part of the locomotive housing the train driver or engineer, the fireman or driver's assistant , and the controls necessary for the locomotive's operation....
of a locomotive
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...
, railcar
Railcar
A railcar, in British English and Australian English, is a self-propelled railway vehicle designed to transport passengers. The term "railcar" is usually used in reference to a train consisting of a single coach , with a driver's cab at one or both ends. Some railways, e.g., the Great Western...
or multiple unit
Multiple unit
The term multiple unit or MU is used to describe a self-propelled carriages capable of coupling with other units of the same or similar type and still being controlled from one driving cab. The term is commonly used to denote passenger trainsets consisting of more than one carriage...
, where the train driver or engine driver
Railroad engineer
A railroad engineer, locomotive engineer, train operator, train driver or engine driver is a person who drives a train on a railroad...
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. These systems range from simple coded track circuits to transponders that communicate with the cab, to communication-based train control systems.
Overview
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 signalRailway signal
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...
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 stop a train.
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
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
in the 1940s. High-speed trains such as those in Japan, Northeastern United States
Northeastern United States
The Northeastern United States is a region of the United States as defined by the United States Census Bureau.-Composition:The region comprises nine states: the New England states of Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont; and the Mid-Atlantic states of New...
, Britain, France, and Germany use such systems in principle, though they are mutually incompatible.
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
Linienzugbeeinflussung
Linienzugbeeinflussung is a cab signalling andtrain protection system used on selected German and Austrian railway linesas well as the AVE in Spain.In Germany, the system is mandatory on all lines where trains exceed speeds of...
, 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
Pennsylvania Railroad
The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....
(PRR) and Union Switch & Signal
Union Switch & Signal
Union Switch and Signal was a supplier of railway signaling equipment, systems and services in Pittsburgh, Pennsylvania. As of January 1, 2009, US&S is known as Ansaldo STS USA.-History:...
(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
London Underground
The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...
Victoria Line
Victoria Line
The Victoria line is a deep-level London Underground line running from the south to the north-east of London. It is coloured light blue on the Tube map...
, and form the basis for the first generation Shinkansen
Shinkansen
The , also known as THE BULLET TRAIN, is a network of high-speed railway lines in Japan operated by four Japan Railways Group companies. Starting with the Tōkaidō Shinkansen in 1964, the network has expanded to currently consist of of lines with maximum speeds of , of Mini-shinkansen with a...
signalling developed by Japan National Railways (JNR).
Hierarchy of cab signal systems
With a traditional signalling system, the engine driver must observe wayside signals and act accordingly, depending on the aspects displayed. This method of operation is susceptible to human failure; if the driver 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 (i.e., 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 and Continuous
There are three classes of cab signal systems:- Wayside signals with intermittent warning and enforcement capabilities
- Continuous cab signal and speed enforcement 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.
Intermittent
Automatic Train Stop
An automatic train stop is a system on a train that will automatically stop a train if certain situations happened to prevent accidents from happening....
(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.
Continuous
Continuous systems, generally known as Automatic Train ControlAutomatic Train Control
Automatic Train Control is a train protection system for railways, ensuring the safe and smooth operation of trains on ATC-enabled lines. Its main advantages include making possible the use of cab signalling instead of track-side signals and the use of smooth deceleration patterns in lieu of the...
(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
Linienzugbeeinflussung
Linienzugbeeinflussung is a cab signalling andtrain protection system used on selected German and Austrian railway linesas well as the AVE in Spain.In Germany, the system is mandatory on all lines where trains exceed speeds of...
in Germany and the TVM
LGV signalling
Transmission Voie-Machine is a form of in-cab signalling originally deployed in France and used on high-speed railway lines. TVM-300 was the first version, followed by TVM-430....
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
ACSES
Advanced Civil Speed Enforcement System is a positive train control cab signaling system developed by PHW and Alstom. The system is designed to prevent train-to-train collisions, protect against overspeed and protect work crews with temporary speed restrictions...
transponder-based speed enforcement systems provides the absolute stop, as well as enforcement of line-speed limits on portions of the Northeast Corridor
Northeast Corridor
The Northeast Corridor is a fully electrified railway line owned primarily by Amtrak serving the Northeast megalopolis of the United States from Boston in the north, via New York to Washington, D.C. in the south, with branches serving other cities...
(NEC).
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.
Mechanical
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 AuthorityNew York City Transit Authority
The New York City Transit Authority is a public authority in the U.S. state of New York that operates public transportation in New York City...
(NYCTA), the Toronto Transit Commission
Toronto Transit Commission
-Island Ferry:The ferry service to the Toronto Islands was operated by the TTC from 1927 until 1962, when it was transferred to the Metro Parks and Culture department. Since 1998, the ferry service is run by Toronto Parks and Recreation.-Gray Coach:...
(TTC) 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
Train stop
Part of a railway signalling system, a train stop or trip stop is a train protection device that automatically stops a train if it attempts to pass a signal when the signal aspect and operating rules prohibit such movement, or if it attempts to pass at an excessive speed.- Basic operation :The...
). If a stop signal is displayed, a trip-arm located next to the running rails becomes "raised." On the bogie
Bogie
A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...
s/trucks of the underground/subway cars, there is an emergency brake
Emergency brake (train)
On trains, the expression emergency brake has several meanings:* The maximum brake force available to the driver/engineer from his conventional braking system, usually operated by taking the brake handle to its furthest postion, through a gate mechanism, or by pushing a separate plunger in the cab*...
valve. If a train overruns a stop signal, the raised arm would strike the emergency brake valve, causing the train's compressed air line to depressurize or be "dumped." The brakes throughout the entire train are automatically applied.
This form of Automatic Train Stop
Automatic Train Stop
An automatic train stop is a system on a train that will automatically stop a train if certain situations happened to prevent accidents from happening....
(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 fieldMagnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
to designate a hazardous condition. The British Rail AWS (Automatic Warning System
Automatic Warning System
The Automatic Warning System is a form of limited cab signalling and train protection system introduced in 1956 in the United Kingdom to help train drivers observe and obey signals. It was based on a 1930 system developed by Alfred Ernest Hudd and marketed as the "Strowger-Hudd" 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
Signal passed at danger
A Signal passed at danger , in British railway terminology, occurs when a train passes a stop signal without authority to do so. It is a term primarily used within the British Railway Industry, although it can be applied worldwide.-Categories of SPAD:...
) 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 since contact between a fast moving train and wayside equipment leads to wear. In the early part of the 20th century, the Great Western RailwayGreat Western Railway
The Great Western Railway was a British railway company that linked London with the south-west and west of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament in 1835 and ran its first trains in 1838...
in Great Britain experimented with an electric system, whereby long bars in the space between the rails became energized with an electric current (supplied from a battery) when the distant signal was clear. See Automatic Warning System.
Inductive
Inductive systemsInductive sensor
An inductive sensor is an electronic proximity sensor, which detects metallic objects without touching them.The sensor consists of an induction loop. Electric current generates a magnetic field, which collapses generating a current that falls asymptotically toward zero from its initial level when...
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 induction loop
Induction loop
Induction loop is a term used to describe an electromagnetic communication- and detection system, relying on the fact that a moving magnet will induce an electrical current in a nearby conducting wire. Induction loops are used for transmission and reception of communication signals, or for...
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 circuitTrack circuit
A track circuit is a simple electrical device used to detect the absence of a train on rail tracks, used to inform signallers and control relevant signals.- Principles and operation :...
, and to continuously transmit signal indications to the train. The coded track circuit systems eliminate the need for specialized beacons.
Examples of coded track circuit systems include the Pennsylvania Railroad standard system
Pulse code cab signaling
Pulse code cab signaling is a form of cab signaling developed in the United States by the Union Switch and Signal corporation for the Pennsylvania Railroad in the 1920s...
, a variation of which is used on the London Underground Victoria Line
Victoria Line
The Victoria line is a deep-level London Underground line running from the south to the north-east of London. It is coloured light blue on the Tube map...
, and one used on the MBTA Red Line. Newer audio frequency
Audio frequency
An audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...
(AF) track circuit systems are used on the Hudson-Bergen Light Rail
Hudson-Bergen Light Rail
The Hudson–Bergen Light Rail is a light rail system in Hudson County, New Jersey, United States. Owned by New Jersey Transit and operated by the 21st Century Rail Corporation, it connects the communities of Bayonne, Jersey City, Hoboken, Weehawken, Union City , and North Bergen.The system began...
(HBLR) and Newark Light Rail
Newark Light Rail
The Newark Light Rail is a light rail system under New Jersey Transit Bus Operations serving Newark, New Jersey. The service consists of two segments, the original Newark City Subway, and the extension to Broad Street station...
. The AF track circuits differ from traditional power-frequency coded track circuits in that they rely more heavily on digital signal processing to transmit and detect information, but can be cheaper and simpler to design and implement.
Typology of cab signalling 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 Pulse code cab signaling Pulse code cab signaling is a form of cab signaling developed in the United States by the Union Switch and Signal corporation for the Pennsylvania Railroad in the 1920s... , Lewistown test installation (1923); Main Lines (including Northeast Corridor Northeast Corridor The Northeast Corridor is a fully electrified railway line owned primarily by Amtrak serving the Northeast megalopolis of the United States from Boston in the north, via New York to Washington, D.C. in the south, with branches serving other cities... ) (1930s), speed enforcement capabilities added in 1950s |
| Trip-stops: New York City Subway New York City Subway The New York City Subway is a rapid transit system owned by the City of New York and leased to the New York City Transit Authority, a subsidiary agency of the Metropolitan Transportation Authority and also known as MTA New York City Transit... , systemwide (current); MBTA Red Line (1970s) |
General Railway Signal (GRS) automatic train stop (ATS) system deployed on Chicago & North Western (CNW), Atchison Topeka & Santa Fe (ATSF), and New York Central (NYC) -- installed circa 1930-1940 | |
| British Rail AWS (Automatic Warning System Automatic Warning System The Automatic Warning System is a form of limited cab signalling and train protection system introduced in 1956 in the United Kingdom to help train drivers observe and obey signals. It was based on a 1930 system developed by Alfred Ernest Hudd and marketed as the "Strowger-Hudd" system... ) |
British Rail TPWS (Train Protection & Warning System) | Audio-Frequency Track Circuit systems installed on Newark City Subway, Hudson-Bergen Light Rail Hudson-Bergen Light Rail The Hudson–Bergen Light Rail is a light rail system in Hudson County, New Jersey, United States. Owned by New Jersey Transit and operated by the 21st Century Rail Corporation, it connects the communities of Bayonne, Jersey City, Hoboken, Weehawken, Union City , and North Bergen.The system began... (New Jersey) |
| 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 East Shore Line East Shore Line East is a commuter rail service operating in southern Connecticut, USA. A fully owned subsidiary of the Connecticut Department of Transportation , SLE provides service seven days a week along the Northeast Corridor from New London west to New Haven, with continuing service to Bridgeport... implementation in 1997 uses ACSES which has the PRR pulse codes at its core; Metro-North Railroad Metro-North Railroad The Metro-North Commuter Railroad , trading as MTA Metro-North Railroad, or, more commonly, Metro-North, is a suburban commuter rail service that is run and managed by the Metropolitan Transportation Authority , an authority of New York State. It is the busiest commuter railroad in the United... and New Jersey Transit Rail Operations New Jersey Transit rail operations New Jersey Transit Rail Operations is the rail division of New Jersey Transit. It provides regional rail service in New Jersey, with most service centered around transportation to and from New York City, Hoboken, and Newark... both implemented similar systems. |
| Magnetic Train Stops on the River Line (New Jersey Transit) River Line (New Jersey Transit) The River Line is a diesel light rail system in New Jersey, United States, that connects the cities of Camden and Trenton, New Jersey's capital. It is operated for New Jersey Transit by the Southern New Jersey Rail Group , which originally included Bechtel Group and Bombardier... |
India Ansaldo Pilot | London Underground Victoria Line (PRR derivative system) |
| SNCF SNCF The SNCF , is France's national state-owned railway company. SNCF operates the country's national rail services, including the TGV, France's high-speed rail network... 's KVB classic line—KVB = Contrôle de Vitesse par Balises Contrôle de vitesse par balises KVB or Contrôle de Vitesse par Balises is a train protection system used on the French railways and in London St Pancras station... (Beacon-based Speed Control) |
SNCF's TGV TGV The TGV is France's high-speed rail service, currently operated by SNCF Voyages, the long-distance rail branch of SNCF, the French national rail operator.... 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), Washington Metro Washington Metro The Washington Metro, commonly called Metro, and unofficially Metrorail, is the rapid transit system in Washington, D.C., United States, and its surrounding suburbs. It is administered by the Washington Metropolitan Area Transit Authority , which also operates Metrobus service under the Metro name... 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 Florida East Coast Railway The Florida East Coast Railway is a Class II railroad operating in the U.S. state of Florida; in the past, it has been a Class I railroad.Built primarily in the last quarter of the 19th century and the first decade of the 20th century, the FEC was a project of Standard Oil principal Henry Morrison... uses a cab signal system with speed enforcement on its main line |
||
| All post-Soviet "Metros" use ALS-ARS (Automatic Locomotion Signaling with Automatic Regulation of Speed = Автоматическая локомотивная сигнализация с автоматическим регулированием скорости) |
Cab signalling systems in the US
Cab signalling in the United States was driven by a 1922 ruling by the Interstate Commerce CommissionInterstate Commerce Commission
The Interstate Commerce Commission was a regulatory body in the United States created by the Interstate Commerce Act of 1887. The agency's original purpose was to regulate railroads to ensure fair rates, to eliminate rate discrimination, and to regulate other aspects of common carriers, including...
(ICC) that required 49 railroads to install some form of automatic train control in one full passenger division by 1925. While several large railroads, including the Santa Fe and New York Central, fulfilled the requirement by installing intermittent inductive train stop devices, the PRR saw an opportunity to improve operational efficiency and installed the first continuous cab signal systems, eventually settling on pulse code cab signalling
Pulse code cab signaling
Pulse code cab signaling is a form of cab signaling developed in the United States by the Union Switch and Signal corporation for the Pennsylvania Railroad in the 1920s...
technology supplied by Union Switch and Signal.
In response to the PRR lead, the ICC mandated that some of the nation's other large railroads must equip at least one division with continuous cab signal technology as a test to compare technologies and operating practices. The affected railroads were less than enthusiastic, and many chose to equip one of their more isolated or less trafficked routes to minimize the number of locomotives to be equipped with the apparatus.
Several railroads chose the inductive loop system rejected by the PRR. These included the Central Railroad of New Jersey
Central Railroad of New Jersey
The Central Railroad of New Jersey , commonly known as the Jersey Central Lines or CNJ, was a Class I railroad with origins in the 1830s, lasting until 1976 when it was absorbed into Conrail with the other bankrupt railroads of the Northeastern United States...
(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" aspects. The CNW went further and 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. The Chicago, Milwaukee, St. Paul and Pacific Railroad
Chicago, Milwaukee, St. Paul and Pacific Railroad
The Milwaukee Road, officially the Chicago, Milwaukee, St. Paul and Pacific Railroad , was a Class I railroad that operated in the Midwest and Northwest of the United States from 1847 until its merger into the Soo Line Railroad on January 1, 1986. The company went through several official names...
had a 3-aspect system operating by 1935 between Portage, Wisconsin
Portage, Wisconsin
Portage is a city in and the county seat of Columbia County, Wisconsin, United States. The population was 10,662 at the 2010 census making it the largest city in Columbia County...
and Minneapolis, Minnesota
Minneapolis, Minnesota
Minneapolis , nicknamed "City of Lakes" and the "Mill City," is the county seat of Hennepin County, the largest city in the U.S. state of Minnesota, and the 48th largest in the United States...
.
As the Pennsylvania Railroad system was the only one adopted on a large scale, it became a de facto national standard, and most installations of cab signals in the current era have been this type. Recently there have been several new types of cab signalling which 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 (SES) employed by New Jersey Transit
New Jersey Transit
The New Jersey Transit Corporation is a statewide public transportation system serving the United States state of New Jersey, and New York, Orange, and Rockland counties in New York State...
on their low density Pascack Valley Line
Pascack Valley Line
The Pascack Valley Line is a commuter rail line operated by the Hoboken Division of New Jersey Transit. The line runs north from Hoboken, New Jersey through Bergen County and into Rockland County, New York, terminating at Spring Valley. Service within New York is operated under contract with...
as a pilot program using a dedicated fleet of 13 GP40PH-2 locomotives. SES used a system of transponder beacons attached to wayside block signals to enforce signal speed. SES was 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 decelerate. SES is in the process of being removed from this line, and is being replaced with CSS.
Amtrak
Amtrak
The National Railroad Passenger Corporation, doing business as Amtrak , is a government-owned corporation that was organized on May 1, 1971, to provide intercity passenger train service in the United States. "Amtrak" is a portmanteau of the words "America" and "track". It is headquartered at Union...
uses the Advanced Civil Speed Enforcement System (ACSES) for its Acela Express
Acela Express
The Acela Express is Amtrak's high-speed rail service along the Northeast Corridor in the Northeast United States between Washington, D.C., and Boston via Baltimore, Philadelphia, and New York...
high-speed rail service on the NEC. ACSES was an overlay to the existing PRR-type CSS and uses the same SES transponder technology to enforce both permanent and temporary speed restrictions at curves and other geographic features. The on-board cab signal unit processes both the pulse code "signal speed" and the ACSES "civil speed", then enforces the lower of the two. ACSES also provides 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.
Positive train control
Positive train controlPositive Train Control
Positive train control is a system of monitoring and controlling train movements to provide increased safety.-Overview:The main concept in PTC is that the train receives information about its location and where it is allowed to safely travel, also known as movement authorities...
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
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...
-Detroit
Detroit, Michigan
Detroit is the major city among the primary cultural, financial, and transportation centers in the Metro Detroit area, a region of 5.2 million people. As the seat of Wayne County, the city of Detroit is the largest city in the U.S. state of Michigan and serves as a major port on the Detroit River...
route in Michigan
Michigan
Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....
and the North American Joint Positive Train Control (NAJPTC) system being testing in Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...
on the Chicago-St. Louis
St. Louis, Missouri
St. Louis is an independent city on the eastern border of Missouri, United States. With a population of 319,294, it was the 58th-largest U.S. city at the 2010 U.S. Census. The Greater St...
route. The ITCS has been in revenue service since 2002 with speeds up to 90 mph (145 km/h). Other systems include Alaska Railroad
Alaska Railroad
The Alaska Railroad is a Class II railroad which extends from Seward and Whittier, in the south of the state of Alaska, in the United States, to Fairbanks , and beyond to Eielson Air Force Base and Fort Wainwright in the interior of that state...
's CAS, CSX
CSX Transportation
CSX Transportation operates a Class I railroad in the United States known as the CSX Railroad. It is the main subsidiary of the CSX Corporation. The company is headquartered in Jacksonville, Florida, and owns approximately 21,000 route miles...
's CBTM, and BNSF
BNSF Railway
The BNSF Railway is a wholly owned subsidiary of Berkshire Hathaway Inc., and is headquartered in Fort Worth, Texas. It is one of seven North American Class I railroads and the second largest freight railroad network in North America, second only to the Union Pacific Railroad, its primary...
's ETMS and V-ETMS manufactured and installed by Wabtec.
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 2 kHz and 20 kHz. Because of the relatively high frequency, the signal quickly attenuatesAttenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
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 they provide the transmission of signal aspects which can change between block boundaries, similar to pulse code cab signals. 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.