The trains I take semi-regularly have a few stops with tiny station platforms, regardless of the length of the train. They will announce something like ‘if you are in the first few cars start walking back’. You better pay attention too or you’ll have to uber from the next station back to where you should’ve gotten off.
Sounds like London's select door service. You can't make the train too long though if it has cars that you can't walk between because then you can't exit at all.