If you’re sentimental about past-their-prime forms of transit, and if you can look past the infelicity that is Amtrak, it’s easy to fall in love with trains. They occupy so many iconic moments in American literature and film, and there are so many songs about them. It’s enough to send you into Songs About Trains, a new musical revue, waxing nostalgic. Then note the subtitle: A Celebration of Labor Through Folk Music. If you’re fond of Woody Guthrie and his ilk and tales of Casey Jones and John Henry, well, the show’s already halfway down the track. But Songs About Trains turns out to be even richer than that. It’s not just songs about trains: It can be seen as the whole damn history of American expansion.