Overlap sharing

[PJW]

Can anybody explain:

What is overlap sharing?
When can we share the overlaps and when we can't?

An overlap is the safety overrun distance that is reserved beyond the route's exit signal, in case a train fails to stop.
Generally in the UK then it is for it's exclusive use, just in case.

However SPADs are actually quite rare and therefore the risk of two simultaneous SPADs in the same area is pretty minimal (disregarding what we call cat B or "technical" SPADs when the signal concerned was legitimately cleared but reverts due to some fault such as loss of point detection or a TC failure. Although driver of train will pass the signal at danger, it is not their fault, nor is there risk since the route was stil set and locked for that mvement due to approach locking / route locking. So two cat B SPADs in an area could well be caused by the same fundamental fault, but zero risk of train collision).

So we could risk assess and say that two signals could share overlap.

This may be "head on"; on a bi-directional line an Up signal might have a 180m beyond it that terminates at the block joint at the Down signal position. If overlaps are not shared clearly there could not be a route set up to that Down signal as it could not have overlap, since reserved for the Up movement. However we could dcide that they could share on the basis that a SPAD at both at same time is not credible.

It could also be "converging". Imagine a double track section of passing loop in a station area becoming a single track with trailing points to the single line portion just beyond loop exit signal.
If an Up train is arriving at loop from double track section, then it would certainly not be sensible to permit a Down through train to pass via the other loop simultaneously, since if first train failed to stop would almost certainly be immediate head on collision. However we could decide that we'd be prepared to allow two Up direction movements, one into one loop from the rnnung line and the other into the other loop from sidings. If either train did SPAD (credible risk) then it wouldn't hit the other.

Generally in the UK we do not share overlaps, but certainly the Western Region did where line speed was low and risks felt reasonable (indeed Reading's new WESTLOCK interlocking commissioned in Dec 2010 implements those principles whilst the existing track layout remains, so it is still something that we do in particular circumstances). Indeed with "risk assessment" to the fore and the traditional sometimes rather black/white rules being revisited, it is certainly possible that such policies will be implemented more widely.

So the question you asked would be just the sort of thing that might come up as a written question in 2011 Exam; anyone feel like writing an answer?

Sara, from your reading of various Australian practices, do any railways there share overlaps?

Any comments from others from "down under" or other parts of the world?