I think the game really needs this feature. When I drive a train, I often (spontaneously) decide to hop off, maybe wait for a train in the opposite direction and drive that one back. Or I just want to have a look around in the station. But the game takes 5 or 10 minutes or so until it asks you if you want to end the service and until then, your train just sits at the platform and waits for you to continue driving. I think it would be really cool if you could just give up the service whenever you want by pressing a button (maybe on the pause menu, if there are no buttons left on the controllers). The AI would continue driving the train then (as on free roam).
It should be possible to both prompt (when you want to give up) and suppress (to avoid when not in the cab out of necessity) the pop up command.