thought I'll share this idea I had...
make a simple diesel engine and throw in one or two of these old lego weight blocks in the front above motor for good measure...then run a lego wire from the 9V motor then strip the other end off to solder onto two leafspring-shaped metal pieces that would sit between the two axles on the dummy bogey to pick up the midrail power.
seem like it could work and wouldn't be too difficult aside to figuring how to clamp the pickup shoes on (may have to stuck it permamently onto the lego rail axles for one)