Instead of using incandescents, use LEDS, which reach full intensity at 3 volts. Radio shack sell two 5 mm white LEDS in a pack, and they are rated for a maximum of 3.5 volts. Hard to believe you'd be running slower than that. You might want to install a voltage regulator, so that if you turned the throttle up you'd keep it at 3 volts. Dave Bodnar has a good article on doing this at
See the section on LED's
you would need a bridge rectifier, so the light stayed on when you changed track polarity, a 3 volt voltage rectifier, some resistors and a big capacitor, to reduce flicker. It's not hard to do.
Alternatively, you could just power LEDs with batteries: although I dislike that option myself you can get a lot of runtime out of 4 AAs