Communications-based train control (CBTC) is a railway signaling system that uses telecommunications between the train 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 system (and other railway systems) are able to reduce headways 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 (ATP) functions, as well as optional automatic train operation (ATO) and automatic train supervision (ATS) functions," as defined in the IEEE 1474 standard.[1]
Background and origin
CBTC is a signalling standard defined by the IEEE 1474 standard.[1] The original version was introduced in 1999 and updated in 2004.[1] 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.[1] The standard therefore does not require the use of moving block railway signalling, but in practice this is the most common arrangement.[2][3][4][5][6][7]
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.[8]
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 30–60kHzfrequency 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.[10][11] 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 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,[11] 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)[1] CBTC solutions that make use of the radio communications.
Main features
CBTC and moving block
CBTC systems are modern railway signaling systems that can mainly be used in urban railway lines (either light 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 [when?]). 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 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.
The safety distance (safe-braking distance) between trains in fixed block and moving block signalling systems
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's border.
In a moving block system as shown in the second figure, the train position and its braking curve 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.[12]
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.[12]
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 requirements. This results in a reduced headway 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.[13] In fact, CBTC is not a synonym for "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 between operating trains, while maintaining the safety 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.[5]
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 service.[14]
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.[15]
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 and improve the traffic capacity compared to manual driving systems.[16][17]
Finally, it is important to mention that the CBTC systems have proven to be more energy efficient than traditional manually driven systems.[15] 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 then 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.[18]
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.[19] 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. 2.4GHz and 5.8GHz) 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 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.[20] 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.[21]
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.[21]
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 architecture of a CBTC system
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 including 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 2,4GHzband is commonly used in these systems (same as WiFi), though other alternative frequencies such as 900MHz (US), 5.8GHz 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 San Francisco or Washington), to complex overlays on existing railway networks carrying more than a million passengers each day and with more than 100 trains (such as lines 1 and 6 in Madrid Metro, line 3 in Shenzhen Metro, some lines in Paris Metro, New York City Subway and Beijing Subway, or the Sub-Surface network in London Underground).[4]
Radio-based CBTC moving block projects around the world. Projects are classified with colours depending on the supplier; those underlined are already into CBTC operation.
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) and those undertaken on completely new lines (Greenfield).
With train attendants (Rovers) who drive trains in the event of a disruption. These train attendants are also on standby between Botanic Gardens and Caldecott stations.
↑ Only radio-based projects using the moving block principle are shown.
↑ This is the number of four-car train sets available. The BMT Canarsie Line runs trains with eight cars.
↑ This is the number of eleven-car train sets available. The IRT Flushing Line runs trains with eleven cars, though they are not all linked together; they are arranged in five- and six-car sets.
↑ Includes a 1.48 km "express bypass" where non-stopping express trains take a different route than stopping local trains.
1 2 This is the number of four- and five- car sets to be equipped with CBTC; they will be linked up in sets of 8 or 10 cars each. The routes that use the Queens Boulevard and Crosstown lines are serviced by trains from Jamaica Yard and East New York Yard.
Railway signalling (BE), or railroad signaling (AE), is a system used to control the movement of railway traffic. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which makes it difficult to quickly stop when encountering an obstacle. In the UK, the Regulation of Railways Act 1889 introduced a series of requirements on matters such as the implementation of interlocked block signalling and other safety measures as a direct result of the Armagh rail disaster in that year.
Automatic train control (ATC) is a general class of train protection systems for railways that involves a speed control mechanism in response to external inputs. For example, a system could effect an emergency brake application if the driver does not react to a signal at danger. ATC systems tend to integrate various cab signalling technologies and they use more granular deceleration patterns in lieu of the rigid stops encountered with the older automatic train stop (ATS) technology. ATC can also be used with automatic train operation (ATO) and is usually considered to be the safety-critical part of a railway system.
Automatic train operation (ATO) is a method of operating trains automatically where the driver is not required or required for supervision at most. Alternatively, ATO can be defined as a subsystem within the automatic train control, which performs any or all of functions like programmed stopping, speed adjusting, door operation, and similar otherwise assigned to the train operator.
Automatic train stop or ATS is a system on a train that automatically stops a train if certain situations occur to prevent accidents. In some scenarios it functions as a type of dead man's switch. Automatic train stop differs from the concept of Automatic Train Control in that ATS usually does not feature an onboard speed control mechanism.
Most trains on the New York City Subway are manually operated. As of 2022, the system currently uses automatic block signaling, with fixed wayside signals and automatic train stops. Many portions of the signaling system were installed between the 1930s and 1960s. Because of the age of the subway system, many replacement parts are unavailable from signaling suppliers and must be custom-built for the New York City Transit Authority, which operates the subway. Additionally, some subway lines have reached their train capacity limits and cannot operate extra trains in the current system.
Standards for North American railroad signaling in the United States are issued by the Association of American Railroads (AAR), which is a trade association of the railroads of Canada, the US, and Mexico. Their system is loosely based on practices developed in the United Kingdom during the early years of railway development. However, North American practice diverged from that of the United Kingdom due to different operating conditions and economic factors between the two regions. In Canada, the Canadian Rail Operating Rules (CROR) are approved by the Minister of Transport under the authority of the Railway Safety Act. Each railway company or transit authority in Canada issues its own CROR rulebook with special instructions peculiar to each individual property. Among the distinctions are:
The Toronto subway uses a variety of signalling systems on its lines, consisting of a combination of fixed block signalling and moving block signalling technologies.
Signaling and operation on the Washington Metro system involves train control, station identification, train signaling, signage, and train length. As with any working railroad, communication between train operators, dispatchers, station personnel and passengers is critical. Failures will result in delays, accidents, and even fatalities. It is therefore important that a comprehensive signal system operated by a central authority be in place. This gives individual train and station operators the information they need to safely and efficiently perform their tasks.
SelTrac is a digital railway signalling technology used to automatically control the movements of rail vehicles. It was the first fully automatic moving-block signalling system to be commercially implemented.
Transmission balise-locomotive is a train protection system used in Belgium and on Hong Kong's East Rail line.
EBICab is a trademark registered by Alstom for the equipment on board a train used as a part of an Automatic Train Control system. Three different families exist, which are technically unrelated.
Pulse code cab signaling is a form of cab signaling technology developed in the United States by the Union Switch and Signal corporation for the Pennsylvania Railroad 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. In its home territory on former PRR successor Conrail owned lines and on railroads operating under the NORAC Rulebook it is known simply as Cab Signaling System or CSS.
The Chinese Train Control System is a train control system used on railway lines in People's Republic of China. CTCS is similar to the European Train Control System (ETCS).
In railway signalling, a moving block is a signalling block system where the blocks are defined in real time by computers as safe zones around each train. This requires both knowledge of the exact location and speed of all trains at any given time, and continual communication between the central signalling system and the train's cab signalling system. Moving block allows trains to run closer together while maintaining required safety margins, thereby increasing the line's overall capacity. It may be contrasted with fixed block signalling systems.
CITYFLO 650 signalling is a CBTC system designed by Bombardier Transportation. It makes use of bi-directional radio communication between trains and wayside equipment, as well as true moving block technology, to control train operation. Trains report their position via radio, and a wayside signalling system provides movement authorities to the trains via a radio link.
Automation of London Underground rolling stock has been partially implemented since the introduction of automatic train operation on the Hainault to Woodford section of the Central line in 1964. It is currently in use on eight lines.
The Système d'aide à la conduite, à l'exploitation et à la maintenance (SACEM) is an embedded, automatic speed train protection system for rapid transit railways. The name means "Driver Assistance, Operation, and Maintenance System".
Modern railway signalling in Thailand on the mainline employs color light signals and computer-based interlocking. The State Railway of Thailand is currently implementing centralized traffic control to link the whole country’s signalling system together using a fiber optic network. This includes recent double-tracking projects for all mainlines extending from Bangkok.
The history of automatic train operation includes key dates for system introductions of different Grade of Automation. The lower grades, such as the German Punktförmige Zugbeeinflussung introduced in 1934 have been available earlier. Higher grades, such as the driverless operation have been introduced almost only in case automated guideway transit.
References
1 2 3 4 5 1474.1–1999 – IEEE Standard for Communications-Based Train Control (CBTC) Performance and Functional Requirements. (Accessed at January 14, 2019).
1 2 Bombardier to Deliver Major London Underground Signalling. Press release, Bombardier Transportation Media Center, 2011. Accessed June 2011
1 2 CITYFLO 650 Metro de Madrid, Solving the capacity challenge.Archived 2012-03-30 at the Wayback Machine Bombardier Transportation Rail Control Solutions, 2010. Accessed June 2011
1 2 "Modernizing the signal system: 2017 subway closures". Toronto Transit Commission. January 18, 2017. Retrieved January 23, 2017. [video position 1:56]Trains will be able to operate as frequently as every 1 minute and 55 seconds instead of the current limit of two and a half minutes. [2:19]When installation is completed along the entire line in 2019, it will allow for as much as 25% more capacity. [2:33]ATC will come online on all of Line 1 in phases by the end of 2019 starting with the portion of Line 1 between Spadina and Wilson stations and with the Line 1 extension into York Region that opens at the end of this year.
↑ Digital radio shows great potential for Rail Bruno Gillaumin, International Railway Journal, May 2001. Retrieved by findarticles.com in June 2011.
↑ IEC 62290-1, Railway applications – Urban guided transport management and command/control systems – Part 1: System principles and fundamental concepts. IEC, 2006. Accessed February 2014
↑ Madrid's silent revolution. in International Railway Journal, Keith Barrow, 2010. Accessed through goliath.ecnext.com in June 2011
1 2 Semi-automatic, driverless, and unattended operation of trains.Archived 2010-11-19 at the Wayback Machine IRSE-ITC, 2010. Accessed through www.irse-itc.net in June 2011
↑ CBTC: más trenes en hora punta.[permanent dead link] Comunidad de Madrid, www.madrig.org, 2010. Accessed June 2011
↑ How CBTC can Increase capacity – communications-based train control. William J. Moore, Railway Age, 2001. Accessed through findarticles.com in June 2011
↑ ETRMS Level 3 Risks and Benefits to UK Railways, pg 19 Transport Research Laboratory. Accessed December 2011
↑ ETRMS Level 3 Risks and Benefits to UK Railways, Table 5 Transport Research Laboratory. Accessed December 2011
↑ ETRMS Level 3 Risks and Benefits to UK Railways, pg 18 Transport Research Laboratory. Accessed December 2011
1 2 CBTC World Congress Presentations, Stockholm, November 2011 Global Transport Forum. Accessed December 2011
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