The layouts I've operated on if a train doesn't finish it's run, the layout owner finishes the run between sessions.Most owners don't get to operate their own layouts and they say this gives them a chance to run their on layout. For me, if I don't finish a run I feel like I've failed to complete the job. I know proto trains don't all finish at midnight; there are some trains running all the time but those are mostly long runs, I think.
One layout I operate on has a local that says you have 16 hours (fast time) to complete the run. It starts at 9:30am. If you do the math, that means you finish by 1:30 am. But the session stops at midnight. So it is scheduled to be out on the road at the end of the session. One of these days I'm going to finish it by midnight; haven't made it yet but I'm getting closer.