Overlap sharing

[PJW]

Peter,

Shared overlaps, are they equivalent to phantom overlaps?

Regards,
Saraswathi Penneru

Not quite. I was actually intending to add to my previous post a diagram to make things clearer and was already thinking of including phantom; I just need to grab some time to do this.

For now though, just words:
If there are 2 opposing signals (113 and 114) and we wish to signal up to each simultaneously, we would need each to have its own overlap distance, say of 180m reserved for its own unique use (assumption of NOT permitting overlap sharing). However we need not necessarily provide a track circuit boundary there; we could make the overlaps both indicate as one track. So instead of tracks AB and AC we would just have track BB of twice the length it would have been otherwise.
This means that occupancy anywhere between the signals would replace both and there is a sharing therefore of the train detection (and thus saves some cost) but if there were two SPADs each could overrun to the end of its own overlap and not actually collide, other than the most gentle touch of the buffers perhaps.
The safety is precisely the same as if two 180m track circuits had been provided, but economy achieved by providing one 360m track instead. Actually arguably slightly improved since, as soon as the first SPAD occurs at 113 and BB becomes occupied, the outer opposing direction signal 116 (i.e that authorising up to 114 which immediately protects) would revert to danger.
In the separate overlap TC instance, track AB would not be naturally proved within the opposing direction overlap so that signal would only revert if the SPAD exceed that length and got as far as AC. Of course nowadays we may well also specifically provide "overrun detection" ; a special control that replaces relevant aspects in the area if a SPAD is detected (e.g. if AB becomes occupied after AA occupied whilst the protecting signal 113 at danger) then the interlocking or control system would take emergency action to mitigate the risk by attempting to stop valid movements in the vicinity.

The difference with a shared overlap is that the signals need only be 180m rather than 360m apart- not only is the train detection section shhared but so also is the overrun distance. It is less safe since the likelihood of collision has been increased ( very very marginally- do you actually consider that it is credible that the two SPADs would occur more or less simultaneously? Once one has occurred it will revert the outer signal so we are looking to risk assess how likely it is that there would at that moment be a train coming which itself wasn't actually going to stop where it was originally supposed to).

When I have time to add diagram, hopefully it should make it clearer.