... Do ceilings really need to be in the game? Why can't the game just check whether or not the above tile has a floor/wall material? That seems like it would be sufficient. I don't see why the floor of one tile can't also act as the ceiling of the tile below.
I've seen a post just like this 2-3 pages ago. I dont feel like checking, but it's virtually verbatim. Did you copy/paste? Despite the redundancy, I agree that this sounds like a logical solution... A surface acting as a floor to the room above, and a ceiling to the room below.
That said, I'd rather have a rail system closer to Transport Tycoon, than Rollercoaster Tycoon. Remember people, this is a band of hardy and highly industrial people! They came to build a military and industrial outpost, not to build a themepark. When the novelty wears off, you'll want the carts to be as efficient as possible.
A loop where a cart goes up a wall doesn't require ceilings, it requires some kind of inverse ramp. The differentiation isn't between "floors" and "ceilings" (which are fundamentally identical in DF), but between a 90-degree angle between a wall and ceiling/floor and a more gradual curve.
It requires inverse ramps? There are gradual curves? Did you play some early build before all others? =O
Well tell me then, you decide to make a loop. How do you stop it from being a closed circle? How do you shift the entrance gently to the left, and the exit gently to the right? Imagine taking a pair of 90 degree turns while hanging upside down at the mercy of the gravity.
More importantly, how do you build constructions (any constructions) on the ceiling without rewriting the entire map format and all the little changes that THAT would break?