Since this thread mostly devolved into a discussion about possible train mission, I thought that, maybe, I could add my own two cents.
If you go through most of complaints about level design in last two games, you can pretty much boil it all down to bottlenecks and dead ends. Basically, in a good Hitman level every new space should create more options to the player regardless of the direction he is moving through the level. Every time the game forced players into one specific locations (field lab, tornado shelter) or forced them to backtrack (school in Marrakesh) they would complain.
And from this standpoint moving train would be a terrible place for a Hitman level since it is mostly choke points with two dead ends on either side.
But then I got thinking. While special navigation accounts for most decision making in Hitman, it is not the only kind of decisions that the game gives you. It should hypothetically be possible to recreate the same kind of branching decision tree for players on a more abstract level.
Basically, what I’m trying to say is that Hitman can build its train level around its adventure-like elements. With navigation out of the way, the game can put a lot more emphasis on interactions with NPCs and the kind of social engineering that is already present in other levels.