N3V Games' Trainz Simulator has been available on the iPad for quite some time, and has enjoyed considerable success, despite not being out favourite game in the world.
Essentially it's the toy train collector's digital dream, providing plenty of opportunities to play around, and even drive, their favourite trains. It takes some work to get used to, but once you do it's all good fun.
Buoyed by that success, the game is now available for download to Android tablets as well. The game looks much the same as the iPad game, so if you're into this kind of thing, then you're sure to have some fun with this.