Thanks to troll queen and Scottro and Nate and Sean and Chimp Rage for this (1, 2, 3, 4). Berlin mini horses take the tubes.
I live in NYC, which has a jillion subway cars flying around this city at all times. I've seen just about anything you can think of on the subways. Yet the owner of the greatest mini pony resource on the Internet has yet to see an awesome tiny horse swiped into the underground, commuting to the station of his choice, and complaining that the tourists are blocking his path to the hay.