I was on a pendo a few years ago where coach B (the coach I was in) was unable to carry passengers. Immediately after departure (around 45 late) the TM came into the carriage and informed everyone they had to move, and we could move into First if we wished ( train not very well loaded, we would have all been able to be seated in Standard elsewhere I am sure.
We were also told over the PA that service on the train would not be up to the usual standard as TM has to stay in Coach B to ensure no one sat in there. Also, at stations the onboard shop had to close so the staff member manning the shop could stay in coach B while the TM did the doors.
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