Well, there's a couple problems with this idea. One is that then the cars would have to some control circuitry, which they don't right now. This is expensive and redundant. Additionally, it would make the trains heavier, which would mean the structure would have to be sturdier, which would again make the thing more expensive. Furthermore, even if you compeletely locked the poly-urethane wheels, they would slide a whole bunch along the steel rails without slowing the train down very much.
It's, overall, a pretty impractical solution.
-----------------
The legend lives!