When a train line uses on-demand stops or even only to ensure an easy way to leave the train a button for requesting a stop and to be automatically moves out off the train would be needed.
The default button for 'use' ("e") is even easy to remember for that (exit).
Some players have problem with leaving the train with the normal moving keys, so this would help to make leaving the train easy and without dangers.
For on-demand stops is currently no way for a player in a train to signal the intention to leave at the next stop.
This would make on-demand tracks easy to construct, even with only ATC-controllers.
It would also need a track counterpart, because otherwise you can't get on.
As counterpart only a ATC-condition is needed (eg IP.. [Player or Pressed]) and and lau command like atc_is_pressed().
The signal for stopping should bound to the player and train - if the player leaves the signal is cleared.
For getting on a train a mesecon button and a switch (transit track/track for stopping) and some gates/lua controller is needed - or the (not implemented) possibility to change an ATC track with a mesecon signal.
Because a player is waiting to enter the train the area is loaded, so there are no problem with mesecons and unloaded areas.