>Not even that. Most lines are already at maximum capacity, and the number of cars is usually dependent on the shortest platform on the line already.

Here in the UK we run trains that are longer than platforms quite frequently. I'm talking about 9 car trains stopping at 2 car platforms here as well, not 9 car trains where the front and rear doors of the first and last carriage don't open.

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 ("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.