redundancy for track circuit

[Peter]

Hi Signalling Professionals:

Recently, I have encountered a track circuit design philosophy which I have doubted the actual benefit that it brings to the railway system.

is it better to implement track circuit with redundant function ? (that's if one track receiver fails, the system will automatic change to other receiver unit for the same track circuit, so the track circuit will work normally as usual). Anyone got ideas ? or has anyone seen this kind of design philosophy implemented for track circuit ?

We saw that the Swiss had done something similar for the Lötschberg Base Tunnel where, for reliability, they completely duplicated the axle counter train detection. Based on the logic that they were SIL4 systems, if the two outputs disagreed on the state of the section, selecting one over the other would be unlikely to result in a genuine wrong side failure since there are many more reasons that the section that was showing occupied would be showing that when it should be clear.

There is the question as to whether you have doubled the wrong side failure probability since you are always selecting the least restrictive and therefore always select the one (if there is one) that wrong side fails. That said, if the chance of a WSF on a SIL4 system is almost negligible, double negligible is still small.

Something to consider is the method by which you select one output or another - how reliable is your mediation system.

Peter