Soggy is right. In "The World's Greatest Roller Coaster Thrill: The Next Generation", the first stop is IOA, and they have an (engineer?) explaining how Hulk is propelled. It is motors...sets of them. I can't remember exactly, but he explains there are a certain number of them on each side, I think at the back, and how they work with a fin. I will have to go home tonight and watch it again, but if you can get this tape, it explains it and alot of other coasters, i.e. Jokers Jinx, Superman-The Escape, etc.