Here’s a slight variation on Britannicus’s idea:
The setting is a sprawling train-station and surrounding areas, including a stationary train that is slowly filling with passengers. For the first fifteen minutes or so, this is all one map.
But after fifteen minutes (or earlier, or later, if 47 intervenes) the train sets off, and you’re either on said train or you’re not.
If you’re not on it, then the level just loses a train and gains an empty track, and you can continue killing targets in and around the station.
If you are on it, then the mission turns into a moving-train level, and you have to exfiltrate by slowing the train down and jumping out the back, slamming on the emergency breaks and getting off, or simply waiting in your seat (a cutscene) so you can depart at the next station.