Rideable would be cool, I was thinking about something to add another way for the player to get around in the train rather than a Minecart. Basically just a fancy looking minecart to go along with the rest of the stuff you made. It would be interesting to see Steve sitting on top of the traincar like a giant.
Oh yeah also, are the yellow hopper carts supposed to show their inventory? The last time I looked in game they do not.