A BCK was quite common as guard accommodation on the various Western Region overnight newspaper trains, which while theoretically not passenger workings would convey various passengers who were either stranded or just turned up hopeful. Principal of these were Naval sailors returning to Plymouth, where, regardless of tickets held, the guard would put any officers in the Firsts and the men in the Seconds. WR staff in the West Country, more than most, had done their time in the Navy, and knew this avoided any embarrassment.
Interesting to hear that Carstairs standby vehicle, which I recall, was in such poor internal condition; I presume if used it would normally only convey the guard. In addition to all the other combinations listed it had smoking and non-smoking compartments for both classes. St Rollox spent quite a lot on its conversion, principally on the dual braking, which only a few vehicles had and which needed finding room in the underframe for both air and vacuum cylinders, and complex double brake rigging. Possibly there was no standard drawing for this and the works drawing office had to devise their own. I wonder how it was ensured the batteries did not go flat. It seems to be just peeking from its normal position into the left hand edge of this photo