Laser Loop at Kennywood was one of these. I'm sure RideMan (among others ;) )will be able to describe it better than I.
As you said, they used a flywheel. Because of inertia ("an object in motion wants to stay in motion barring outside forces"), a heavy wheel that's spinning rapidly is very hard to stop (gyroscopes use that principle).
So if you get a huge flywheel spinning, and then attach a cable to it, and attach that cable to a train, the train is going to get yanked, because it doesn't provide NEARLY enough resistance to stop the flywheel.
Now, I don't know much about the mechanism for engaging the train TO the drivewheel. Anyone want to take a shot at that?
-------------
--Greg
http://www.pobox.com/~gregleg/
*** This post was edited by GregLeg on 2/16/2001. ***