Bug 1 : the furnace minecart takes 1 piece of coal to go on forever
Bug 2 : minecart chains can't go on turns and go up without the minecarts separating
Bug 3 : When the furnace minecart stops, the smoke is still being emitted
I don't know any other way to make trains/subways without using the furnace minecart, and subways are underground so they will fit in with the cave update.
There's most likely more bugs I haven't found.