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 systems (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.1474.1–1999 – IEEE Standard for Communications-Based Train Control (CBTC) Performance and Functional Requirements. (Accessed at January 14, 2019).
Background and origin
CBTC is a signalling standard defined by 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.
As a result, 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 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, now Hitachi Rail) for the Bombardier Automated Rapid Transit (ART) systems in Canada 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 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 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.
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 ). 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.
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.
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 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.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 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 0On-sight, with no automation GoA 1Manual, with a driver controlling all train operations. GoA 2Semi-automatic operation (STO), starting and stopping are automated, but a driver who sits in the cab operates the doors and drives in emergencies GoA 3Driverless train operation (DTO), starting and stopping are automated, but a crew member operates the doors from within the train GoA 4Unattended 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.CITYFLO 650 Metro de Madrid, Solving the capacity challenge. 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 service.Madrid's silent revolution. 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 trains. 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 and improve the traffic capacity compared to manual driving systems.CBTC: más trenes en hora punta. 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.ETRMS Level 3 Risks and Benefits to UK Railways, pg 19 Transport Research Laboratory. Accessed December 2011
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.