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)
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!) :)