Communication-based train control

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Communication-based train control (CBTC) is an automated control system for railways that ensures the safe operation of rail vehicles using data communication between various control entities that make up the system.

CBTC is a modern successor of the traditional railway signaling systems which provide a limited control using track circuits, interlockings, and signals.

CBTC technology is part of the same convergence engineering field as the automotive traffic automation systems. Even though the principles of control between the 2 transportation domains are quite different they both are part of the telematics: a CBTC system is based on basic principles from both the automation and telecommunication.

CBTC is the culmination point of a long parallel history of two important technologies that stimulated the development of each other and marked profoundly the development of our society in the last century: railways, telecommunications.

There are multiple generations of CBTC systems based on multiple variations and configurations depending on the providers and operators.

A CBTC can provide different levels of railway automation, grouped in the following major functionalities:

- ATP: Automatic Train Protection;
- ATO: Automatic Train Operation;
- ATS: Automatic Train Supervision.

CBTC systems range in complexity based on the functionality provided from CBTC systems that provide only the ATP functions to CBTC that provide both the ATP and ATO functions and CBTC that on top of that provide also the ATS functions.

All CBTC systems operate in the same basic way. An off board computer can track all trains operating on track linked to the computer. This allows trains to be spaced closer together than on a manually controlled line. The computer sends information to the computers aboard the train.

This differs from normal Automatic Train Operation (ATO) systems in that the train is actually controlled by the on board computers. Older ATO systems, such as San Francisco’s BART, use a system where the off board computers control the train.

Based on the media used for communication the CBTC are categorized in:
- radio-based;
- rail-based;
- loop-based.

The older of the three, rail-based, has been in use in a few systems. Train control signals are sent through the tracks to the train. RF, which has yet to be used on an in service heavy rail Metro application, transmits the signal from wayside equipment. Washington DC's Metro is completely CBTC based, and now MTA New York City Subway in the United States. The New York City Subway 14th Street-Canarsie Local route (line L) is nearing the completion of tests on the new control system.

The CBTCs deployed on transit systems as the SRT line from Toronto, and SkyTrain from Vancouver are based on loop communication.