This is a very significant point which explains the widespread disappearance of overnight passenger trains, be they seated or sleepers. In the past, the railway carried a lot of overnight mail/parcel/newspaper traffic in vans which were fully compatible with passenger stock, i.e. had steam or electric heating connections and were capable of the same speeds. So...it was no skin off BR's nose to stick one or two passenger carriages on to provide a bit of capacity, as it didn't really cost them anything and even if only one or two people used it - frequently the case - they got a bit of extra cash. Always popular with enthusiasts if a Mk 1 compartment vehicle was provided - armrests up, blinds down, lightbulbs out the window, a welcome few hours' kip!
The first blow came with sectorisation, as it was a major obstacle to the running of mixed trains. The next blow was the widespread disappearance of mail/parcels traffic. The result was a huge reduction in the number of obscure long-distance overnight workings.