Uh, holy stuff.
Barns gave me an idea of how it might work.
Instead of a vehicle, why not a bot, or some sort of bot (that can/has the ability to tilt if going on incline)
Basically, player mounts train, and instead of them driving it, certain bricks with nodes, like pathnodes, will guide the train to the next node and so forth.
Speed can be changed, turning can be somewhat dynamic, basically, it's train without vehicle physics.