So, basically, this is an internal "subway system" and the tubes are equivalent to subway tunnels. Whereas I'm using a model more approximating the little pneumatic tube systems you still see at banks sometimes (and which used to be everywhere, in the pre-computer days). I'll stick with mine for now... but I can see how your suggestion would work. The tubes aren't really doing anything except keeping the cars separate from the habitable spaces in yours. In mine, the tubes are "the rails" and are what constrain the cars. Different approaches, both workable.