Track Circuit and Train Detection: An Analogy

This is a summary of the article back to basics by IRSE [1], describing the functions of track detections, with addition of opinion by the writer.

In railway signalling, the signaller needs to know whether a track is occupied, i.e. whether there is a train on it. An accident could happen if the signaller sends a train to a track that is occupied. Hence the development of track detection. Two of the most well used track detection is track circuit and axle counter.

Track circuit (TC)

Basically it connects two rails (in a series) and feed power into it. When a train, with metal wheels passes through it, it will ‘short circuit’ the connections, implying the track is occupied by a train.

Since it is based on the concept of ‘shorting’ the circuit, various factors can affect the detection, mainly in two ways:

1. The track circuit is ‘shorted’ by other materials and shows track occupied, even though it isn’t -e.g. track is flooded, ballast is too wet (low ballast resistance), worn out insulated rail joints (IRJ), faulty grounding

2. Track circuit is NOT shorted, even when train is occupying the track – rails are rusty (preventing current flow), wheels are rusty, wheels contaminated (by oil, leaves), too low feed from TC

Axle counters (AX)

An inductive metal detector head counts number of wheels/axles passing, and compares it with a neighbouring axle counter. Missing wheels implies that the train is still between these two neighbouring axle counters. It’s concept and implementation is relatively simpler than track circuit.

TC vs AX

As axle counter has more edge, it is easier to word as; Axle Counters benefit vs TC

1. Immune to traction current interference

2. impervious to rail head or ballast resistance

3. no limitation on section length

4. no impact on track bonding/earthing (traction power)

5. no requirement for IRJ

6. can overlay other detection system, even other AX counter – for brown field project

With all these benefits, TC is still the most used type of track detection in the UK. Why? One possibility is the cost, i.e. the cost of replacing all the TC with axle counters. It must be said that axle counters comes with its own set of failures, but that is not within this article.

However I personally assumed it is not only the factor of replacement cost, as the failures in these track circuits’ also present a loss in income for the operation [2], but also the factor of structural inertia or resistance to change. Track circuit is widely used and has allowed signalling to ‘track’ the movement of the train remotely, without needing a clear line of sight for the signaller.

An analogy to this that I can think of is the advance of USB, from the previous USB A to the now USB C. USB C has various advantages; it can charge devices faster, it can reverse charge, can be used for power as well as data/video/audio, and most importantly, can be plugged easily. However I have too many equipment around the house that uses USB A that I simply can’t be bothered to replace the devices and/or the plug. Maybe when the devices are all broken, then I’ll migrate the plugs and/or buy adapters to USB C.

I think the rail industry has a similar view, in which, they will still be using it until it is broken, or a really easy (read: cheap) solutions come along.

References

1. Train detection – the basics – David Fenner https://webinfo.uk/webdocssl/irse-kbase/ref-viewer.aspx?type=noindex&RefNo=-1460017217&NextPrevious=YES

2. Condition Monitoring of Audio Frequency Track Circuits – P. F. Weston, J. Chen, E. Stewart and C. Roberts https://ieeexplore.ieee.org/document/4730847