Sure it's doable. I remember a Matchbox-type car racing set I had when I was young. It had a hand-cranked motor that turned a couple of foam brushes pretty fast. It gave the cars a push every time they came around. You could actually get them going pretty fast.
It's not K'nex. If it was, it would actually work, whereas this probably won't. I bought one of the Six Flags coasters before off of ebay and it is a piece of crap. Doesn't make the circuit and the track is too rigid.
This is the Six Flags Cool Coasters system that came out a year or so ago. It's basically like a Hot Wheels/Matchbox type car that runs on coaster track. Looks like they've changed the colors of the plastic and have remarketed a set or two. Not a bad idea really, especially since these kits are dirt cheap ($20 or so)
Proud Member of The Out-Of-Town Coaster Weirdos
Queen: Yah, I think it was Mattel. From the pictures of the toy coaster, it looks like it uses a similar system to launch the trains (but probably not hand cranked - kids today would have a fit if they had to do it by hand!) :)
I'm still trying to figure out why you would want a Mr. Six bobbelhead. Go buy a real bobbelhead like Kinzel or Eisner. ;)
Me? I'm dishonest, and a dishonest man you can always trust to be dishonest. Honestly. It's the honest ones you want to watch out for, because you can never predict when they're going to do something incredibly... stupid.