It indeed does run by gravity, pure and simple. The trains are infact very light, as heavier trains would cause more friction on the pin bearings (like those on model railway rolling stock). They could build the trains heavy with miniature high speed roller bearings, but if not anything else, they'd atleast increase the cost significantly, and (the anything else) the bearings might infact have more operational friction than the plain pins, at this scale.
I've not got one of the models myself and don't plan on getting one, but that's how I understand things to be, by what the Coasterdynamix guys themselves say.
I'm planning on building my own scatchbuilt woodie model, 1:16 scale. My desired 80' lift will be five actual feet high . I've just bought a circular table saw to turn sheets of 6mm plywood into ½" strips, for the bent uprights (scale 8x4s, as many old woodies used. I'm told CCI usually used 6x3 uprights and as a consequence, the parks with them are suffering problems with the bolt holes causing cracks.. funny that they used smaller posts for coasters which use the heaviest PTC trains yet). Anyway, going off topic there.. just a bit of trivia I suppose.