Only those coffins with a reservation…..
They would probably stand as much chance as a bike reservation has.
Standing a coffin up would allow it to travel in the passenger compartments of the train, standing alongside everyone else who cannot get a seat
I can remember the
GWR▸ (original GWR) covered coffin trolley at Paddington Station in the 1970's / 80's the mechanical fitters in the station Maintainenance section had a 3 monthly inspection and maintenance of it to check the brake, wheel bearing, door latches and towing bar.