Choo Choo Jonny: they are used for two main reasons (1) carry new auto/vehicles from manufacturing point to distribution points for dealers, (2) on the Amtrak Auto Trains.
The auto racks that are being made today seem to be O gauge [traditional O] I just measured one of my old O gauge ones and from top of shelf to top of car about 4 1/4" high. "O" scale of course would be higher.