Perhaps I may be permitted to answer some of the questions Chris poses in his drawing
The headshunt ground disc (signal 7) is pulled for moves over the crossover using the outer end of platform 4 only, such moves also see platform 4's starter or shunt disc pulled. Shunt moves are permitted past it into the goods yard with the signal on. Some railways would have used a yellow disc for this move.
The signal indicated adjacent to goods yard siding 4 (signal 21) controls exit from the fan of points onto the Up departure road. Most departing freights use this route rather than the crossover using the end of platform 4. It has a small arm indicating that it does not apply to passenger trains. The blue board at its foot would have carried the local instructions to staff as to when they can pass this signal at danger - as indeed one does to access siding 4.
The smaller arms on the gantry are indeed calling on arms, but not all railways marked these with a C.
The top entry on the smaller theatre light should be 3 as, passing under this signal one can only access platforms 3, 4 and the yard
One "secret", is that the operators do not need to know the meaning of every signal, as the schedule tells them exactly which signals and points to pull (and the order in which to pull them).
I hope this helps
Tony
https://www.flickr.com/photos/75514026@N03/albums/72157649004452418/with/20344807871/