In the UK it's quite normal, I think, for them to tell people who want to get off at stop X that they have to be in carriages Y-Z because the platform isn't lon

In the UK it's quite normal, I think, for them to tell people who want to get off at stop X that they have to be in carriages Y-Z because the platform isn't long enough for them to open the doors of other carriages. There seems to be a Wikipedia page on the general topic: https://en.wikipedia.org/wiki/Selective_door_operation https://en.wikipedia.org/wiki/Selective_door_operation ("Selective Door Operation enables trains to call at a station where the platform is shorter than the train.") It's my understanding that the capacity of the UK train network is mostly limited by the granularity of the signalling.