Silly as it may sound if you haven't tried it, take a few brushes and a dremel tool and experiment. Drill a 1/16" hole in the center of each brush face and see what happens. Also try using your dremel with a cutoff wheel to cut a slot in the face - try a horizontal slot, a vertical slot (or two), cut an H in it, etc. Any of these will boost the power of your motor, and a little experimentation will tell you which works best for you and your track.