YLP Wrote:Thanks Alexgoei. I checked that the points & T.C. that I mentioned is not incorporated in your answer nor suggested by PJW. However I want to mak sure why they are not required to be incorporated. Maybe some description from the O.S. Nock book has become out-dated?
It is a good question.
What Alex did not tell you that he and I did discuss this issue when side by side last May whilst enjoying a Singapore Sling cocktail with the layout diagram spread out on Raffles long bar (who knows the photo may yet appear in IRSE News....).
These points are not just flank but actually trapping- guards against unattended vehicles rolling out of the siding if is it is windy, the handbrake not applied and the air brake leaks off. I would set and lock and detect 251 flank as the errant vehicle could endanger the route from 113 signal post to 129 signal post; I wouldn't worry about 238 as the errant vehicle would only intrude into the overlap and be "going away" so it would only be a danger to our train if it itself SPADs 129 and thus "unreasonable" to provide locking. It is a bit arguable and different places around the UK (historic regional practices and indeed "vintage" date) vary so I wouldn't be vehement- having already discussed with Alex when he reattempted the Control Tables without it I didn't feel the need to raise it as it is not "definitely wrong".
Indeed the likelihood of a vehicle being detached and moving by itself is much less now than historically:
1) originally freight trains were unbraked- these are now a thing of the past in the UK. Automatic air brakes should stop vehicle if it becomes attached- only if a vehicle is left for weeks is it likely for the pressure gradually to be lost. This is a Goods Loop to allow overtaking and temporarily holding an entire train prior to a margin to push back into the sidings; it is not somewhere that is likely to hold a wagon / loco-less train for an extended period. Nowadays certain Goods Loops like this are actually signalled to passenger standards and not given traps at all; the risk depends upon intended usage.
2) Also almost all freight in UK is composed of "block trains" running in basically fixed formation, not lots of individual wagons frequently coupled, uncoupled and shunted in portions and then rejoined, the coupling being by a person hooking a chain over a hook. Hence the likelihood of wagons becoming detached is rather lower than historically.
3)The other thing that we do nowadays with power operated points such as 251 is to make them auto-normalise after no longer required reverse (or alternatively sound an alarm to encourage the signaller to do so if left reverse after a train has passed over them. No guarantee that are actually normal, but again rather less likelihood of them being reverse so that a wagon can escape.
Hence like much in railway signalling, as is other engineering, is a balance of risks and a compromise between apparent safety and reliability & operating convenience / flexibility. In reality you
PJW