Pat,
It's basically a tractor (like on a payloader) with rail wheels, you could drive it on the road, put it on the rails, lift the road wheels and use to to move cars around.
There was an article in the May 1985 MR about scratch-building a trackmobile in HO. IIRC they used a flea to power it and styrene for the body, and wheels from a tractor kit or something like that for the road wheels. In N scale the motor and gear box would probably cause the biggest problems, those critters are SMALL.