I’ve been trying to get my head around the signalling arrangements at stations governed by more than one signal-box. I think (famous last words) that I understand the principles, but I’d be grateful for any help putting them into practice!
The first two diagrams below show what I think would be a fairly normal set-up in which the box in rear (“West”) has its starter signal slotted with the home signal of the box in advance (“East”). Black sections show the block controlled by each box, and green sections are station limits.
Is it correct that the East box (in advance) has no actual block under control, and thus that “Is Line Clear?” posed from West to East relates only to the ¼-mile overlap in advance of the East home signal? Or to put it another way, that West could run a train up to the East home without consulting East at all?
In my mind the West starter / East home signal is slotted at the post just like a home/distant signal, thus the starter lever at West could be pulled but the arm would remain “on”. In that scenario would the West signaller be required to observe rule 39(a) and caution a train at their own home signal, and would this rely on their attentiveness to the actual position of the starter/home arm or maybe a reminder device? Or am I completely wrong and there would actually be some kind of mechanical interlock between the boxes themselves so that the starter lever couldn't be pulled until East had pulled their home lever? Maybe this is situation specific, thinking about it.
(In the diagrams I’ve showed slotted signals with the arm showing the most “urgent” restriction in the horizontal position but I’m not sure if that's correct. The distinction in the RSSB guidance previously shared on this forum seems to be between “slotted by” another box and “slotted at” another box which is clear as mud to me!)
If I’ve got things right so far, then I guess that in the above example (considering only the West box now) that a loco headed “up” from West to East could run around its train under the control of the ground signals A and B and head off back down-line without troubling the signaller at East.
On the other hand it wouldn’t be permitted for the loco to pass the West starter (aka East home) before reversing unless there was a special instruction to allow working in the wrong direction back into East’s (zero-length!) block, and presumably special interlocking / acceptance levers? (I guess this is what acceptance levers are for, I’ve seen them on SB diagrams but never understood their purpose without being able to cross-reference the local rules of course.)
Finally before my head explodes, would a scenario like this be practical? Again the signals shown as “active” are controlled from the West box, and although it looks contrived in isolation(!), in effect the platform road here is just like a bidirectional platform in a larger station (or the “back road” at e.g. Ely).
As I see it the East signaller can safely pull off his starter (which is actually a slot on both arms of West’s down home as shown – I guess one lever would suffice for this) and thus give a train the right away into the reversible platform! Part of me feels like this must be wrong as there “ought” to be a token or suchlike before a train can be released into a bidirectional track, but is it actually all ok because that track is entirely within West’s station limits and the signal doesn’t actually clear until West is good and ready and has also pulled a lever?
Hopefully some more knowledgeable than me can help here, and if so many thanks in anticipation! I’ve been a longtime lurker on here and am always in awe of the expertise on offer…
Sam