Bite-sized Revision Notes

[PJW]

I have several queries on the Notes:

4. Bite 18 Points (Point 6 bullet 3)
- Can you explain more why the route lock only needs to maintain after berth TC clear, but not require us to write all T.C from start signal till the berth TC?

I think you have misunderstood; Time of Operation Locking is subtly different from route locking.
T of Op is applies to certain facing points in a swinging overlap; these are therefore typically free to move from one lie to another whilst a train is routed up to the exit signal, provided that both of the overlaps are currently available (otherwise the swinging overlap locking would prevent). However we wouldn't want them to swing just at the moment when a train SPADed if there was then a signficant risk of the train reaching them in mid-stroke.
Hence if the points a are "close" (you may find some old books quote 20m, my experience nowadays when we have to calculate on the site specifics is that 40m is more like the limit) then T of Op is applied. This reflects the fact that until train reaches commencement of the dead track circuit they would otherwise be free and there would be insufficient time for them to complete any throw that had been started just prior to the SPAD being detected.

The entry therefore lists the "priming" condition- after route used but this itself does not lock. The lock is applied when the berth (and the issue is that it is ONLY the actual berth) track circuit is occupied; (i.e. locks the points early so as to be ready for a SPAD should it occur) and then once that same track has been occupied long enough (i.e. the risk of a SPAD has passed) then release the locking again.

Of course you often do get T of Op on the same points as swinging overlap locking; the T of Op ALWAYS locks N>R AND R>N whereas obviously any swinging O/L locking is only in one direction (although there may be the same routes but with different swinging overlap conditions for the converse swing). The points could potentially be trailers (or locked facers) in the overlap of some other routes yet obviously not those that apply T of Op.