I have two nephews who love trains. One of them dresses as an engineer as often as possible, and both know arcana about trains that I could never in a million years master. Neither of my kids ever really went through that obsessive stage (though Nick does know an awful lot about dragons...), but I remember my brother going through a dinosaur stage that was somewhat similar. I'm always impressed by the way kids absorb knowledge about the things that interest them, and it's been fun to watch my nephews with the trains.
But it may be a good thing they don't live in New York; the subway system offers a whole new level of obsessiveness, at least for some kids. This is a fabulous picture-book tale of two boys and the subway--check it out!