I wouldn't risk it. As the others say, feed too much into the motors and you do risk frying them.
You would be best to use a 12v controler/transformer
However if you don't want to buy a new controler you could construct a 'limiter' circuit. This will be placed after the controler but before the tracks. It limits the amount of voltage going to the tracks, so even with the controler wacked up to full you will get no more than 12v.