The story takes place in the future, and in the United States, so it can really be whatever kind of train it needs to be in order to reach some decent speed. The only requirements is that it needs to run off of coal power, and since pretty much all steam-powered trains in the United States have been replaced by better trains, I imagine this is a pretty old locomotive.
I see it as a freight train though. Aside from the engine car it really only needs two other cars attached to it, but figure at least one or two more cars for a bunch of people to pile into.
Cargo is somewhere between 10 and 100 people, a bunch of guns, and a 12,000 pound bomb.
It needs to travel roughly 50 miles. The faster it can go the better.
So I guess more specifically, what kind of steam-powered train would this need to be? How fast could it go? How long would it take it to reach top speed?
As far as terrain goes... I'm not really sure to be honest. I imagine it'd vary a lot. Might have to go up and down some hills, but there'd probably be a lot of flat space too.