A friend and I worked together in modifing a “old style can motor” Bachmann Plymouth mechanism to use as a small low profile power truck for On30 (or HO) use. The center axle was cut down, the end axles were lenghtened and F9 N scale wheels were installed. A frame was made from doulble sided PC board and bushing and pickups from brass. More detailed pictures are here : http://www.2guyz.info/Forums/viewtopic/t=2067/start=0.html
Here it is compaired to the famous GE44T power truck.
The wheels are about 13 inches.
Enjoy, Fred