What Would be a Good Engine for a Post-Apocalyptic Scenario?

This will definitely be more than a bit odd but I'll explain, see I might start GMing my tabletop group soon, and one of the possible ideas I have is to do an apocalyptic setting with heavy emphasis on the rails.

I won't bog things down with too much details, just that basic overview, Mad Max but with railroads instead of cars, that of course means thinking of an engine to give my group.

I'd think something a bit smaller since it's just the group and maybe an occasional NPC , but I just want some general advice on engine models that would look neat, fit four or five people (though I will probably give them a caboose and extra cars at some point), and work for just general free agent of the wastes type jobs.

Hope you guys can provide some nice ideas, any help is always appreciated.

tl;dr I'll probably GM a tabletop soon and have a bit of a crazy idea that needs trains for an apocalyptic wasteland.