I'm just curious if this is possible. To be specific, I'll be having Athearn streamlined passenger cars. It might save me a little money, and electronic components are much easier to get here in the Philippines. Best of all, it might be fun.
Mike, I think that the Rapido cars are lit with LEDs, but they're powered by "button" batteries.
You'd probably find that the easiest way to power yours, as I don't think that the Athearn cars come with metal wheelsets. The battery-powered lights look better, too, as there's no flickering over bad or dirty trackwork.
Battery power will also work better because led's will burn out at 12 volts. If you use the small button type batteries, you can hide them in some of the under floor details so they don't interfere with installing full interiors.
The Rapido cars use a magnetic wand to control the on/off, but you could install a micro-switch on the underbody somewhere, and operate it with tweezers without removing the car from the track.