Pulse code cab signaling
Encyclopedia
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....
in the 1920s. The 4-aspect system widely adopted by the PRR and its successor railroads has become the dominant railroad cab signaling system in North America with versions of the technology also being adopted in Europe and rapid transit systems.
History
Interstate 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...
issued a ruling requiring trains to be equipped with automatic train stop technology to operate at 80 mph or greater. The Pennsylvania Railroad decided to use this as an opportunity to implement a signaling technology that could improve both safety and operational efficiency by displaying a signal continuously in the locomotive cab. The task was assigned to Union Switch and Signal corporation, the PRR's preferred signal supplier.
The first test installation between Sunbury
Sunbury, Pennsylvania
Sunbury is a city in Northumberland County, Pennsylvania, United States. The city is located on the east bank of the Susquehanna River, just downstream of the confluence of its main and West branches. The population was 9,905 at the 2010 census...
and Lewistown, PA
Lewistown, Pennsylvania
Lewistown is a borough in and the county seat of Mifflin County, Pennsylvania, United States. It lies along the Juniata River, northwest of Harrisburg. The number of people living in the borough in 1900 was 4,451; in 1910, 8,166; and in 1940, 13,017. The population was 8,998 at the 2000 census,...
in 1923 used the tracks as an inductive
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
loop coupled to the locomotive’s receiver. The system had two 60 Hz signals. The break-sensing “track” signal was fed down one rail towards the oncoming train and crossed through its wheels, returning in the other rail. The pickup just ahead of the wheels would sum the approaching current from one side with the returning current on the other. The externally returned ”loop” signal was fed into and out of the mid tap of a resistor across each end of the track circuit. The pickup would sum the approaching current on each side as it carried on past to the far end of the track. This signal was shifted 90 degrees from the other. The signals were applied one or both continuously to give Approach or Clear aspects while no signal was a Restricting aspect. The test installation eliminated wayside block signals, and trains relied solely on cab signals.
For its next installation, on the Northern Central line between Baltimore, MD and Harrisburg, PA
Harrisburg, Pennsylvania
Harrisburg is the capital of Pennsylvania. As of the 2010 census, the city had a population of 49,528, making it the ninth largest city in Pennsylvania...
in 1926 (1927?), the PRR tested another variation of cab signals which dropped the loop signal and switched to 100 Hz for the track signal. The pivotal change was that now it would come on above Restricting merely as a carrier and 1.25 to 3 Hz on-off pulsing of it would be used as a code to convey the aspects. The presence of the carrier alone was not meaningful, no pulsing would still mean a Restricting aspect. This new system allowed four signal aspects, Restricting, Approach, Approach (next signal at) Medium (speed) and Clear. Initially the cab signaling system only acted as a 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....
where the engineer would have to acknowledge 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
Automatic 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...
which enforced the rulebook speed associated with each cab signal (Clear = No Restriction, Approach Medium = 45 mph, Approach = 30 mph, Restricting = 20 mph).
Over time the PRR installed cab signals over much of its eastern system from Pittsburgh to Philadelphia, New York to Washington. This system was then inherited by Conrail and 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...
and various commuter agencies running on former PRR territory such as SEPTA and 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...
. Because all trains running in cab signal territory had to be equipped with cab signals, most locomotives of the aforementioned roads were equipped 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.
Technical overview
Basic operation
Pulse code cab signals work by sending metered pulses along an existing AC 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 :...
operating at some chosen carrier frequency. The pulses are detected via induction by a sensor hanging a few inches above the rail before the leading set of wheels. The codes are measured in pulses per minute and for the 4-aspect PRR system are set at 180 ppm for Clear, 120 ppm for Approach Medium, 75 ppm for Approach and 0 for Restricting. The pulse rates are chosen to avoid any one rate being a multiple of another leading to reflected harmonics causing false indications.
The system is 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. Trains with an insufficient number of axles will not short out all of the cab signal current so that following trains might receive an incorrect aspect. Trains of this type must be given absolute block protection to the rear.
Where DC and 25 Hz AC electrification
Electrification
Electrification originally referred to the build out of the electrical generating and distribution systems which occurred in the United States, England and other countries from the mid 1880's until around 1940 and is in progress in developing countries. This also included the change over from line...
co-exist, the standard 100 Hz frequency is changed to 91Hz (next available M-G set frequency). This avoids even harmonics created by the return rail's DC traction current offsetting the AC return sine wave in the same rail.
Enhancements for higher speeds
70 years after pulse code cab signals had been introduced, the 4 speed design was found to be insufficient for speeds not envisioned when the system was designed. The two most pressing problems were the use of high speed turnouts, which allowed trains to take a diverging route faster than the normal 30 or 45 mph covered by the existing cab signals. The introduction of Amtrak's Acela ExpressAcela 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...
service with its 135 mph to 150 mph maximum speeds would also exceed the capabilities of the legacy signaling system and its 125 mph design speed.
To address the problem and avoid a complete rebuild of the signaling system, impair lower speed service, break backwards compatibility with existing cab signals or place to high a reliance on the human operator, an overlay pulse code system was devised for use on Amtrak's Northeast Corridor. By operating with a different carrier frequency of 250 Hz, additional pulse codes could be sent to the train without interfering with legacy 100 Hz codes. By carefully designing the overlay codes, backwards compatibility could be maintained so that any train unable to detect the new codes would never receive a signal more favorable than had it would otherwise detect. In addition to the use of 250 Hz codes, a 5th, 270ppm code was incorporated from rapid transit
Rapid transit
A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...
and Long Island Rail Road use.
The mapping of codes to speeds is as follows.
| 100 Hz code | 250 Hz code | Cab Signal Aspect | Cab Signal Speed | Notes |
| 180 | 180 | Clear | 150 mph | Legacy Units get Clear (125 mph) |
| 180 | --- | Clear | 125 mph | Original PRR System Code |
| 270 | 270 | Clear | 100 mph | Used for high density signaling. |
| 270 | --- | Cab Speed | 60 mph | Used for high density signaling. Compatible with LIRR ASC System |
| 120 | 120 | Cab Speed | 80 mph | Used for most high speed turnouts. Legacy Units get Approach Medium. |
| 120 | --- | Approach Medium | 45 mph | Original PRR System Code |
| 75 | --- | Approach | 30 mph | Original PRR System Code |
| 0 | --- | Restricting | 20 mph | Failsafe state. Original PRR System Code |
Trains with the ability to get the 250 Hz codes get upgraded speeds on track sections with speeds greater than 125 mph and on 80 mph high speed turnouts. Trains without simply travel at the slower speeds. The 270ppm code does break backwards compatibility with the 4-code system, but is only in use around New York Penn Station as part of a high density signaling upgrade. The 270ppm code and 60 mph speed were chosen to be compatible with the cab signals installed on the Long Island Rail Road trains that also use Penn Station.
Cab display unit
Cab signals are presented to the locomotive by means of a cab signal display unit. The earliest CDUs consisted of miniature signals of the type visible along the track, back lit by light bulbs. These could be found in both color light and position light varieties depending on the railroad's native signaling system. Modern CDUs on passenger trains are often integrated with the speedometerSpeedometer
A speedometer is a gauge that measures and displays the instantaneous speed of a land vehicle. Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards. Speedometers for other vehicles have specific names...
, as cab signals now serve a speed control function. On trains equipped with automatic train control
Automatic 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...
functionality failure to properly acknowledge a restrictive cab signal change results in a 'penalty brake application', as does failure to observe the cab signal speed limit.
Current lines using the 4-aspect PRR cab signal system
- Amtrak Northeast CorridorNortheast CorridorThe 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...
Line - Amtrak Harrisburg LineKeystone CorridorThe Keystone Corridor is a Federal Railroad Administration "designated high speed corridor" with a 349-mile railroad line between Philadelphia, Pennsylvania and Pittsburgh, Pennsylvania with a top speed of...
- Amtrak Springfield LineNew Haven-Springfield LineThe New Haven–Springfield Line or Hartford Line is a railroad line owned by Amtrak from New Haven, Connecticut north to Springfield, Massachusetts. As a branch of the Northeast Corridor at New Haven, it is served by approximately seven daily Regional round trips, some continuing from New Haven to...
- Amtrak Shore Line (no waysides New Haven to Providence)
- Norfolk Southern Pittsburgh LineMain Line (Pennsylvania Railroad)The Main Line of the Pennsylvania Railroad was a rail line in the U.S. state of Pennsylvania, connecting Philadelphia with Pittsburgh via Harrisburg...
- Norfolk Southern Port Road Line
- Norfolk Southern Conemaugh LineConemaugh LineThe Conemaugh Line is a rail line owned and operated by the Norfolk Southern Railway in the U.S. state of Pennsylvania. The line runs from Conpitt Junction northwest and southwest to Pittsburgh along a former Pennsylvania Railroad line...
(no waysides) - Norfolk Southern Morrisville Line (no waysides)
- Norfolk Southern Fort Wayne LineFort Wayne LineThe Fort Wayne Line and Fort Wayne Secondary is a rail line owned and operated by the Norfolk Southern Railway , Chicago, Fort Wayne and Eastern Railroad , and CSX Transportation in the U.S. states of Pennsylvania, Ohio, and Indiana...
(Conway Yard to Alliance, OhioAlliance, OhioAlliance is a city in Stark and Mahoning counties in the U.S. state of Ohio. The population was 22,322 at the 2010 census. Alliance's nickname is "The Carnation City", and the city is home to the University of Mount Union....
, no waysides) - Norfolk Southern Cleveland Line (Alliance, Ohio to Cleveland, OhioCleveland, OhioCleveland is a city in the U.S. state of Ohio and is the county seat of Cuyahoga County, the most populous county in the state. The city is located in northeastern Ohio on the southern shore of Lake Erie, approximately west of the Pennsylvania border...
, no waysides) - CSX Boston SubdivisionBoston SubdivisionThe Boston Subdivision is a railroad line owned by CSX Transportation in the U.S. state of Massachusetts. The line runs from Boston west to near Springfield along a former New York Central Railroad line. Its east end is at Amtrak's Northeast Corridor at Back Bay Station, over which CSX has...
(no waysides) - CSX Hudson SubdivisionHudson SubdivisionThe Hudson Subdivision is a railroad line owned by CSX Transportation in the U.S. state of New York. The line runs from Poughkeepsie north along the east shore of the Hudson River to Rensselaer and northwest via Albany and Schenectady to Hoffmans along a former New York Central Railroad line...
- CSX Landover Subdivision
- CSX RF&P Sub (formerly using RF&P CSS system at 60 Hz)
- NJT All Lines
- Metro-North All "East of Hudson" Lines (no waysides)
- MBTA Old ColonyOld Colony RailroadThe Old Colony Railroad was a major railroad system, mainly covering southeastern Massachusetts and parts of Rhode Island. It operated from 1845 to 1893. Old Colony trains ran from Boston to points such as Plymouth, Fall River, New Bedford, Newport, Providence, Fitchburg, Lowell and Cape Cod...
Lines (no waysides) - SEPTA Main LineLansdale/Doylestown LineThe Lansdale/Doylestown Line is a SEPTA Regional Rail line connecting Center City Philadelphia to Doylestown in Bucks County.-Route:The Lansdale-Doylestown segment of the R5 line utilizes what is known as the "SEPTA Main Line", a four-track line that has been owned by SEPTA since 1983...
(Center City to Lansdale) - SEPTA Airport Line
- SEPTA Chestnut Hill West LineChestnut Hill West LineThe Chestnut Hill West Line , is a route of the SEPTA Regional Rail system. The route serves the northwestern section of Philadelphia with service to Germantown, Mount Airy, and Chestnut Hill...
- SEPTA Media/Elwyn LineMedia/Elwyn LineThe Media/Elwyn Line is a SEPTA Regional Rail line running from Center City Philadelphia west to Elwyn in Delaware County.Originally known as the Media/West Chester Branch, service was truncated on September 19, 1986 from West Chester to its current terminus at Elwyn. Service expansion beyond Elwyn...
- SEPTA West Trenton LineWest Trenton Line (SEPTA)The West Trenton Line is a SEPTA Regional Rail line connecting Center City Philadelphia to West Trenton, New Jersey.-Route:The West Trenton Line connects Center City, Philadelphia with the West Trenton section of Ewing, New Jersey...
- SEPTA Fox Chase LineFox Chase LineThe Fox Chase Line is a route of the SEPTA Regional Rail system.Originally known as the Fox Chase/Newtown Branch, service was truncated in January 1983 from Newtown to its current terminus in Philadelphia at Fox Chase due to unreliable train equipment and low ridership...
Related North American pulse code systems
- Long Island Rail RoadLong Island Rail RoadThe Long Island Rail Road or LIRR is a commuter rail system serving the length of Long Island, New York. It is the busiest commuter railroad in North America, serving about 81.5 million passengers each year. Established in 1834 and having operated continuously since then, it is the oldest US...
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 AuthorityMetropolitan Transportation Authority (New York)The Metropolitan Transportation Authority of the State of New York is a public benefit corporation responsible for public transportation in the U.S...
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. The additional codes are used to display speeds of 50/60 and 60/70 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 lineBNSF Railway LineThe BNSF Railway Line is a commuter rail line in the United States, provided by Metra and operated by the BNSF Railway in Chicago and its surrounding suburbs...
to Aurora, IllinoisAurora, IllinoisAurora is the second most populous city in the U.S. state of Illinois, and the 112th largest city in the United States. A suburb of Chicago, located west of the Loop, its population in 2010 was 197,899. Originally founded within Kane County, Aurora's city limits have expanded greatly over the past...
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 IslandRock Island DistrictThe Rock Island District is a commuter rail line operated by Metra from Chicago, Illinois, United States, southwest to Joliet. While Metra does not specifically refer to any of its lines by a particular color, the timetable accents for the Rock Island District line are printed in "Rocket Red"...
Automatic Train Control: Another PRR based cab signal system remnant from the Rock Island. The system is in service on the Metra Rock Island District between Blue Island and Joliet.
- 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 operationAutomatic train operationAutomatic train operation ensures partial or complete automatic train piloting and driverless functions.Most systems elect to maintain a driver to mitigate risks associated with failures or emergencies....
schemes. Rapid transit systems are typically failsafe with a 0 code mandating a complete stop. 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 MetrorailMiami MetroMiami Metro may refer to:*South Florida metropolitan area, also known as the Miami metropolitan area*Metrorail , the heavy rail transit system serving the immediate Miami area...
. Pulse-code technology on rapid transit lines has generally been supplanted by Audio-Frequency cab signals.
- MTA Staten Island Railway 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 50 ppm close-in code.
European pulse code systems
- RS4 CodiciRS4 CodiciRS4 Codici is a train protection system used in Italy. The term is an abbreviation of Ripetizione Segnali a 4 codici ....
is the legacy cab signaling system used in Italy. The system makes use of 0, 75, 120, 180 and 270 ppm codes using a 50 Hz current.
- ALSNALSNALSN is a train control system meaning Continuous Automatic Train Signalling used widely on the main lines of the ex-Soviet states . It uses modulated pulses inducted into rails similar to the Italian RS4 Codici and American Pulse Code Cab Signaling...
(Continuous Automatic Train Signalling) is a legacy system used in the ex-Soviet states (Russian Federation, Ukraine, Belarus, Latvia, Lithuania, Estonia). The system makes use of pulses at 0, 25, 50 and 75 ppm. Extensions in the ALS-ARS automatic train operation system encode with 75, 125, 175, 225, 275 ppm.
- Continuous Automatic Warning SystemContinuous Automatic Warning SystemThe Continuous Automatic Warning System is a form of cab signalling and train protection system used in Ireland to help train drivers observe and obey lineside signals....
is the cab signaling systems in Ireland. The system makes use of 0, 50, 120 and 180 ppm codes using a 50 Hz carrier current. Additional codes are used on some rapid transitRapid transitA rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...
lines.
- Automatische treinbeïnvloedingAutomatische treinbeïnvloedingAutomatische TreinBeïnvloeding or ATB is a Dutch train protection system developed in the 1950s. Its installation was spurred by the Harmelen train disaster of 1962....
is the DutchNetherlandsThe 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...
cab signaling system. It makes use of 0, 75, 120 and 180 ppm codes, supplemented by an inductive train stop system for speeds under 25 mph.
- London UndergroundLondon UndergroundThe 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 LineVictoria LineThe 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...
uses US&S supplied pulse code cab signals to implement its Automatic train operationAutomatic train operationAutomatic train operation ensures partial or complete automatic train piloting and driverless functions.Most systems elect to maintain a driver to mitigate risks associated with failures or emergencies....
system. Codes used are 420, 270, 180 and 120 ppm.