communications-based train control
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Communications-based train control (CBTC) is a railway signaling system that uses
telecommunications Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...
between the
train A train (from Old French , from Latin">-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ... , from Latin , "to pull, to draw") is a series of connected vehicles th ...
and track equipment for traffic management and infrastructure control. CBTC allows a train's position to be known more accurately than with traditional signaling systems. This can make railway traffic management safer and more efficient.
Rapid transit Rapid transit or mass rapid transit (MRT) or heavy rail, commonly referred to as metro, is a type of high-capacity public transport that is generally built in urban areas. A grade separation, grade separated rapid transit line below ground su ...
systems (and other railway systems) are able to reduce
headway Headway is the distance or duration between vehicles in a transit system. The ''minimum headway'' is the shortest such distance or time achievable by a system without a reduction in the speed of vehicles. The precise definition varies depending on ...
s while maintaining or even improving safety. A CBTC system is a "continuous, automatic train control system utilizing high-resolution train location determination, independent from track circuits; continuous, high-capacity, bidirectional train-to-wayside data communications; and trainborne and wayside processors capable of implementing
automatic train protection Automatic train protection (ATP) is the generic term for train protection systems that continually check that the speed of a train is compatible with the permitted speed allowed by signalling, including automatic stop at certain signal aspects ...
(ATP) functions, as well as optional automatic train operation (ATO) and automatic train supervision (ATS) functions," as defined in the
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is an American 501(c)(3) organization, 501(c)(3) public charity professional organization for electrical engineering, electronics engineering, and other related disciplines. The IEEE ...
1474 standard.1474.1–1999 – IEEE Standard for Communications-Based Train Control (CBTC) Performance and Functional Requirement

(Accessed at January 14, 2019).


Background and origin

CBTC is a signalling standard defined by the
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is an American 501(c)(3) organization, 501(c)(3) public charity professional organization for electrical engineering, electronics engineering, and other related disciplines. The IEEE ...
1474 standard. The original version was introduced in 1999 and updated in 2004. The aim was to create consistency and standardisation between digital railway signalling systems that allow for an increase in train capacity through what the standard defines as high-resolution train location determination. The standard therefore does not require the use of moving block railway signalling, but in practice this is the most common arrangement.


Moving block

Traditional signalling systems detect trains in discrete sections of the track called ' blocks', each protected by signals that prevent a train entering an occupied block. Since every block is a fixed section of track, these systems are referred to as fixed block systems. In a moving block CBTC system the protected section for each train is a "block" that moves with and trails behind it, and provides continuous communication of the train's exact position via radio, inductive loop, etc.Digital radio shows great potential for Rai

Bruno Gillaumin, International Railway Journal, May 2001. Retrieved by findarticles.com in June 2011.
As a result, Bombardier Transportation, Bombardier opened the world's first radio-based CBTC system at San Francisco airport's automated people mover (APM) in February 2003. A few months later, in June 2003,
Alstom Alstom SA () is a French multinational rolling stock manufacturer which operates worldwide in rail transport markets. It is active in the fields of passenger transportation, signaling, and locomotives, producing high-speed, suburban, regional ...
introduced the railway application of its radio technology on the Singapore North East Line. CBTC has its origins in the loop-based systems developed by Alcatel SEL (later
Thales Thales of Miletus ( ; ; ) was an Ancient Greek philosophy, Ancient Greek Pre-Socratic philosophy, pre-Socratic Philosophy, philosopher from Miletus in Ionia, Asia Minor. Thales was one of the Seven Sages of Greece, Seven Sages, founding figure ...
, now Hitachi Rail) for the Bombardier Automated Rapid Transit (ART) systems in
Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
during the mid-1980s. These systems, which were also referred to as transmission-based train control (TBTC), made use of inductive loop transmission techniques for track to train communication, introducing an alternative to track circuit based communication. This technology, operating in the 30–60  kHz
frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
range to communicate trains and wayside equipment, was widely adopted by the metro operators in spite of some electromagnetic compatibility (EMC) issues, as well as other installation and maintenance concerns (see SelTrac for further information regarding transmission-based train-control). As with new application of any technology, some problems arose at the beginning, mainly due to compatibility and interoperability aspects.CBTC Projects

www.tsd.org/cbtc/projects, 2005. Accessed June 2011.
CBTC radios: What to do? Which way to go

Tom Sullivan, 2005. www.tsd.org. Accessed May 2011.
However, there have been relevant improvements since then, and currently the reliability of the radio-based communication systems has grown significantly. Moreover, it is important to highlight that not all the systems using
radio communication Radio is the technology of telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transm ...
technology are considered to be CBTC systems. So, for clarity and to keep in line with the state-of-the-art solutions for operator's requirements, this article only covers the latest moving block principle based (either true moving block or virtual block, so not dependent on track-based detection of the trains) CBTC solutions that make use of the
radio communications Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected t ...
.


Main features


CBTC and moving block

CBTC systems are modern railway signaling systems that can mainly be used in urban railway lines (either
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
or heavy) and APMs, although it could also be deployed on commuter lines. For main lines, a similar system might be the European Railway Traffic Management System ERTMS Level 3 (not yet fully defined ). In the modern CBTC systems the trains continuously calculate and communicate their status via radio to the wayside equipment distributed along the line. This status includes, among other parameters, the exact position, speed, travel direction and braking distance. This information allows calculation of the area potentially occupied by the train on the track. It also enables the wayside equipment to define the points on the line that must never be passed by the other trains on the same track. These points are communicated to make the trains automatically and continuously adjust their speed while maintaining the
safety Safety is the state of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk. Meanings The word 'safety' entered the English language in the 1 ...
and comfort ( jerk) requirements. So, the trains continuously receive information regarding the distance to the preceding train and are then able to adjust their safety distance accordingly. From the signalling system perspective, the first figure shows the total occupancy of the leading train by including the whole blocks which the train is located on. This is due to the fact that it is impossible for the system to know exactly where the train actually is within these blocks. Therefore, the fixed block system only allows the following train to move up to the last unoccupied
block Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
's border. In a moving block system as shown in the second figure, the train position and its
braking curve A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. Background ...
is continuously calculated by the trains, and then communicated via radio to the wayside equipment. Thus, the wayside equipment is able to establish protected areas, each one called Limit of Movement Authority (LMA), up to the nearest obstacle (in the figure the tail of the train in front). Movement Authority (MA) is the permission for a train to move to a specific location within the constraints of the infrastructure and with supervision of speed. End of Authority is the location to which the train is permitted to proceed and where target speed is equal to zero. End of Movement is the location to which the train is permitted to proceed according to an MA. When transmitting an MA, it is the end of the last section given in the MA. It is important to mention that the occupancy calculated in these systems must include a safety margin for location uncertainty (in yellow in the figure) added to the length of the train. Both of them form what is usually called 'Footprint'. This safety margin depends on the accuracy of the odometry system in the train. CBTC systems based on moving block allows the reduction of the safety distance between two consecutive trains. This distance is varying according to the continuous updates of the train location and speed, maintaining the
safety Safety is the state of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk. Meanings The word 'safety' entered the English language in the 1 ...
requirements. This results in a reduced
headway Headway is the distance or duration between vehicles in a transit system. The ''minimum headway'' is the shortest such distance or time achievable by a system without a reduction in the speed of vehicles. The precise definition varies depending on ...
between consecutive trains and an increased transport capacity.


Grades of automation

Modern CBTC systems allow different levels of automation or grades of automation (GoA), as defined and classified in the IEC 62290–1.IEC 62290-1, Railway applications – Urban guided transport management and command/control systems – Part 1: System principles and fundamental concept

IEC, 2006. Accessed February 2014
In fact, CBTC is not a synonym for " Automatic train operation, driverless" or "automated trains" although it is considered as a basic enabler technology for this purpose. There are four grades of automation available: * GoA 0 - On-sight, with no automation * GoA 1 - Manual, with a driver controlling all train operations. * GoA 2 - Semi-automatic Operation (STO), starting and stopping are automated, but a driver who sits in the cab operates the doors and drives in emergencies * GoA 3 - Driverless Train Operation (DTO), starting and stopping are automated, but a crew member operates the doors from within the train * GoA 4 - Unattended Train Operation (UTO), starting, stopping and doors are all automated, with no required crew member on board


Main applications

CBTC systems allow optimal use of the railway infrastructure as well as achieving maximum capacity and minimum
headway Headway is the distance or duration between vehicles in a transit system. The ''minimum headway'' is the shortest such distance or time achievable by a system without a reduction in the speed of vehicles. The precise definition varies depending on ...
between operating trains, while maintaining the
safety Safety is the state of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk. Meanings The word 'safety' entered the English language in the 1 ...
requirements. These systems are suitable for the new highly demanding urban lines, but also to be overlaid on existing lines in order to improve their performance.''CITYFLO'' 650 Metro de Madrid, Solving the capacity challeng

Bombardier Transportation Rail Control Solutions, 2010. Accessed June 2011
Of course, in the case of upgrading existing lines the design, installation, test and commissioning stages are much more critical. This is mainly due to the challenge of deploying the overlying system without disrupting the
revenue In accounting, revenue is the total amount of income generated by the sale of product (business), goods and services related to the primary operations of a business. Commercial revenue may also be referred to as sales or as turnover. Some compan ...
service.Madrid's silent revolutio

in International Railway Journal, Keith Barrow, 2010. Accessed through goliath.ecnext.com in June 2011


Main benefits

The evolution of the technology and the experience gained in operation over the last 30 years means that modern CBTC systems are more reliable and less prone to failure than older train control systems. CBTC systems normally have less wayside equipment and their diagnostic and monitoring tools have been improved, which makes them easier to implement and, more importantly, easier to maintain.Semi-automatic, driverless, and unattended operation of train

IRSE-ITC, 2010. Accessed through www.irse-itc.net in June 2011
CBTC technology is evolving, making use of the latest techniques and components to offer more compact systems and simpler architectures. For instance, with the advent of modern electronics it has been possible to build in redundancy so that single failures do not adversely impact operational availability. Moreover, these systems offer complete flexibility in terms of operational schedules or timetables, enabling urban rail operators to respond to the specific traffic demand more swiftly and efficiently and to solve traffic congestion problems. In fact, automatic operation systems have the potential to significantly reduce the
headway Headway is the distance or duration between vehicles in a transit system. The ''minimum headway'' is the shortest such distance or time achievable by a system without a reduction in the speed of vehicles. The precise definition varies depending on ...
and improve the headway#Capacity, traffic capacity compared to manual driving systems.CBTC: más trenes en hora punt

Comunidad de Madrid, www.madrig.org, 2010. Accessed June 2011
Finally, it is important to mention that the CBTC systems have proven to be more energy efficient than traditional manually driven systems. The use of new functionalities, such as automatic driving strategies or a better adaptation of the transport offer to the actual demand, allows significant energy savings reducing the power consumption.


Risks

The primary risk of an electronic train control system is that if the communications link between any of the trains is disrupted, all or part of the system might have to enter a failsafe state until the problem is remedied. Depending on the severity of the communication loss, this state can range from vehicles temporarily reducing speed, coming to a halt or operating in a degraded mode until communications are re-established. If communication outage is permanent, some sort of contingency operation must be implemented which may consist of manual operation using absolute block or, in the worst case, the substitution of an alternative form of transportation. As a result, high availability of CBTC systems is crucial for proper operation, especially if such systems are used to increase transport capacity and reduce headway. System redundancy and recovery mechanisms must then be thoroughly checked to achieve a high robustness in operation. With the increased availability of the CBTC system, there is also a need for extensive training and periodical refresh of system operators on the recovery procedures. In fact, one of the major system hazards in CBTC systems is the probability of human error and improper application of recovery procedures if the system becomes unavailable. Communications failures can result from equipment malfunction, electromagnetic interference, weak signal strength or saturation of the communications medium. In this case, an interruption can result in a service brake or emergency brake application as real time situational awareness is a critical safety requirement for CBTC and if these interruptions are frequent enough it could seriously impact service. This is the reason why, historically, CBTC systems first implemented radio communication systems in 2003, when the required technology was mature enough for critical applications. In systems with poor line of sight or spectrum/bandwidth limitations a larger than anticipated number of transponders may be required to enhance the service. This is usually more of an issue with applying CBTC to existing transit systems in tunnels that were not designed from the outset to support it. An alternate method to improve system availability in tunnels is the use of leaky feeder cable that, while having higher initial costs (material + installation) achieves a more reliable radio link. With the emerging services over open ISM radio bands (i.e. 2.4 GHz and 5.8 GHz) and the potential disruption over critical CBTC services, there is an increasing pressure in the international community (ref. report 676 of UITP organization, Reservation of a Frequency Spectrum for Critical Safety Applications dedicated to Urban Rail Systems) to reserve a frequency band specifically for radio-based urban rail systems. Such decision would help standardize CBTC systems across the market (a growing demand from most operators) and ensure availability for those critical systems. As a CBTC system is required to have
high availability High availability (HA) is a characteristic of a system that aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period. There is now more dependence on these systems as a result of modernization ...
and particularly, allow for a graceful degradation, a secondary method of signaling might be provided to ensure some level of non-degraded service upon partial or complete CBTC unavailability.ETRMS Level 3 Risks and Benefits to UK Railways, pg 1

Transport Research Laboratory. Accessed December 2011
This is particularly relevant for brownfield implementations (lines with an already existing signalling system) where the infrastructure design cannot be controlled and coexistence with legacy systems is required, at least, temporarily.CBTC World Congress Presentations, Stockholm, November 201

Global Transport Forum. Accessed December 2011
For example, the BMT Canarsie Line in New York City was outfitted with a backup automatic block signaling system capable of supporting 12 trains per hour (tph), compared with the 26 tph of the CBTC system. Although this is a rather common architecture for resignalling projects, it can negate some of the cost savings of CBTC if applied to new lines. This is still a key point in the CBTC development (and is still being discussed), since some providers and operators argue that a fully redundant architecture of the CBTC system may however achieve high availability values by itself. In principle, CBTC systems may be designed with centralized supervision systems in order to improve maintainability and reduce installation costs. If so, there is an increased risk of a single point of failure that could disrupt service over an entire system or line. Fixed block systems usually work with distributed logic that are normally more resistant to such outages. Therefore, a careful analysis of the benefits and risks of a given CBTC architecture (centralized vs. distributed) must be done during system design. When CBTC is applied to systems that previously ran under complete human control with operators working on sight it may actually result in a reduction in capacity (albeit with an increase in safety). This is because CBTC operates with less positional certainty than human sight and also with greater margins for error as worst-case train parameters are applied for the design (e.g. guaranteed emergency brake rate vs. nominal brake rate). For instance, CBTC introduction in Philly's Center City trolley tunnel resulted initially in a marked increase in travel time and corresponding decrease in capacity when compared with the unprotected manual driving. This was the offset to finally eradicate vehicle collisions which on-sight driving cannot avoid and showcases the usual conflicts between operation and safety.


Architecture

The typical architecture of a modern CBTC system comprises the following main subsystems: # Wayside equipment, which includes the interlocking and the subsystems controlling every zone in the line or network (typically containing the wayside ATP and ATO functionalities). Depending on the suppliers, the architectures may be centralized or distributed. The control of the system is performed from a central command ATS, though local control subsystems may be also included as a fallback. # CBTC onboard equipment, including ATP and ATO subsystems in the vehicles. # Train to wayside communication subsystem, currently based on radio links. Thus, although a CBTC architecture is always depending on the supplier and its technical approach, the following logical components may be found generally in a typical CBTC architecture: * Onboard ATP system. This subsystem is in charge of the continuous control of the train speed according to the safety profile, and applying the brake if it is necessary. It is also in charge of the communication with the wayside ATP subsystem in order to exchange the information needed for a safe operation (sending speed and braking distance, and receiving the limit of movement authority for a safe operation). * Onboard ATO system. It is responsible for the automatic control of the traction and braking effort in order to keep the train under the threshold established by the ATP subsystem. Its main task is either to facilitate the driver or attendant functions, or even to operate the train in a fully automatic mode while maintaining the traffic regulation targets and passenger comfort. It also allows the selection of different automatic driving strategies to adapt the runtime or even reduce the power consumption. * Wayside ATP system. This subsystem undertakes the management of all the communications with the trains in its area. Additionally, it calculates the limits of movement authority that every train must respect while operating in the mentioned area. This task is therefore critical for the operation safety. * Wayside ATO system. It is in charge of controlling the destination and regulation targets of every train. The wayside ATO functionality provides all the trains in the system with their destination as well as with other data such as the dwell time in the stations. Additionally, it may also perform auxiliary and non-safety related tasks, for instance alarm/event communication and management, or handling skip/hold station commands. * Communication system. The CBTC systems integrate a digital networked radio system by means of antennas or leaky feeder cable for the bi-directional communication between the track equipment and the trains. The 2,4
GHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...
band is commonly used in these systems (same as
WiFi Wi-Fi () is a family of wireless network protocols based on the IEEE 802.11 family of standards, which are commonly used for Wireless LAN, local area networking of devices and Internet access, allowing nearby digital devices to exchange data by ...
), though other alternative frequencies such as 900 MHz ( US), 5.8 GHz or other licensed bands may be used as well. * ATS system. The ATS system is commonly integrated within most of the CBTC solutions. Its main task is to act as the interface between the operator and the system, managing the traffic according to the specific regulation criteria. Other tasks may include the event and alarm management as well as acting as the interface with external systems. * Interlocking system. When needed as an independent subsystem (for instance as a fallback system), it will be in charge of the vital control of the trackside objects such as switches or signals, as well as other related functionality. In the case of simpler networks or lines, the functionality of the interlocking may be integrated into the wayside ATP system.


Projects

CBTC technology has been (and is being) successfully implemented for a variety of applications as shown in the figure below (mid 2011). They range from some implementations with short track, limited numbers of vehicles and few operating modes (such as the airport APMs in Heathrow or Gatwick, to complex overlays on existing railway networks carrying more than a million passengers each day and with more than 100 trains (such as
London Underground The London Underground (also known simply as the Underground or as the Tube) is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England. The Undergro ...
Jubilee Line The Jubilee line is a London Underground line that runs between in suburban north-west London and in east London, via the West End of London, West End, South Bank and London Docklands, Docklands. Opened in 1979, it is the newest line on the ...
and
Northern Line The Northern line is a London Underground line that runs between North London and South London. It is printed in black on the Tube map. It carries more passengers per year than any other Underground linearound 340million in 2019making it the bu ...
,
MTR The Mass Transit Railway system, known locally by the initialism MTR, is a rapid transit system in Hong Kong and the territory's principal mode of Rail transport in Hong Kong, railway transportation. Operated by the MTR Corporation (MTRCL), ...
Tuen Ma Line, Klang Valley Mass Rapid Transit Kajang Line and Putrajaya Line).Bombardier to Deliver Major London Underground Signallin

Press release, Bombardier Transportation Media Center, 2011. Accessed June 2011

Despite the difficulty, the table below tries to summarize and reference the main radio-based CBTC systems deployed around the world as well as those ongoing projects being developed. Besides, the table distinguishes between the implementations performed over existing and operative systems ( Brownfield (software development), brownfield) and those undertaken on completely new lines ( Greenfield).


List


Notes and references


Notes


References


Further reading


Argenia Railway Technologies SafeNet CBTC

Thales SelTrac(R) CBTC
{{Railwaysignalling Train protection systems Telematics Railway signalling block systems