Tracking trains using track circuits has been the conventional wisdom for the past 150 years. No single invention in the history of rail has contributed more towards safety then the track circuit. This simple invention is the foundation for block signalling and the primary method of tracking trains through a mass transit system.
However, over the past 30 years CBTC technologies have rendered this foundational component obsolete. Therefore, how does a CBTC system track trains without track circuits?
Tracking Trains in Conventional Signalling
A track circuit is a hardwired circuit connected directly to the rail. Each track circuit area is called a block. If there is no train in this block, a power source energizes a relay indicating “no occupancy”. If a train enters the block it will short circuit the track circuit - through its axle - de-energizing the relay indicating “occupancy”.
The connection is direct from the track circuit to the relay logic in the equipment room. This design does not require the designer to verify the reported occupancy; it’s taken as gospel. The track circuit provides complete certainty and this is the nature of conventional signalling using track circuits.
Tracking Trains in CBTC
A communication based (CBTC) train tracking system uses an indirect path to determine the location of the train (or occupancy to use a conventional term).
The train borne unit reports a position via a communication medium to the wayside unit, which utilizes the information to ensure there is a safe separation between trains. Unfortunately, there is no certainty that this position report is correct due to the non-vital nature of the communication medium.
Therefore, a CBTC system introduces variables that are not present in conventional track circuit based systems such as:
- The reported position may have been corrupted by the communication medium.
- Time delays induced by the communication medium.
- Total loss of communication.
These are unique problems to CBTC and their solution will not be found using conventional methodologies.
To address these challenges a CBTC system must verify the position report before accepting it by performing plausibility checks; a position report must be plausible before it is accepted.
Plausibility checks are a series of tests to determine the validity of the position report. There are three types of checks the wayside unit should perform:
- Plausibility Window Test - Does the position report fall within a plausible window?
- Communication Test - Is the train to wayside communication active?
- Travel Direction Test - Is the reported position in the correct direction of travel?
Tracking trains in a CBTC system is very different from a conventional track circuit based system. Conventional systems provide certainty whereas CBTC systems are inherently uncertain. As a result CBTC systems must perform extra checks to ensure the position reported is accurate before accepting it.
Other systems may have additional tests but I believe these are three basic tests that all CBTC systems perform in some fashion.
Note 1: IEEE 1474.1 is not clear on this topic. Section 22.214.171.124 (CBTC train location/train speed determination) talks about establishing “the location, speed and travel direction” of each train but this is discussed from the train perspective and not from the wayside perspective.
Note 2: The purpose of the information contained in this post is to describe high level concepts only. It is not intended to be used in a design. If anyone uses these concepts, it is their responsibility to independently review and verify their design through a proper engineering process and to ensure the system and safety requirements are addressed. The author bears no responsibility.