TPWS

[PJW]

Thanks guys for the reply, I have been working away so only just managed to read the replies, but some interesting things there indeed, I am also wondering if you have anything which explains the circuitry for TPWS, and how it works in conjunction with signals, in particular a failure of TPWS.

Not very readily. Basically the TPWS is energised with signal at Red and there is a level of proving that has an output relay called the VCR. The signal irear either requireds the VCR up or the forward signal proved off (i.e. no need to stop the train as can't SPAD a signal that isn't at red!).
Does get more complicated where there are OSS to prove as well; also for approach released signals where a train is permitted t approach a signal t red with a failed TPWS, but only if the only reason why the forward signal is held to red is awaiting an approach release condition- it is said to be "ready to clear".

In retrofit the failure of the TPWS is indicated as if the signal red lamp had failed; in newer installations this is not done but a specific indication lamp is provided- steady blue when no fault, flashing blue if fault