Fred,
Consider that most prototype cranes had the vertical supports connected. The motors were visible at wheel height. My first attempt at moving the crane involved magnets. I had changed the die cast wheels to sintered steel hoping rare earth magnets under the wheels would move it. That didn't work very well so I made a structure to bridge the supports and put additional magnets in the bridge. That did work but the movement was not smooth. Here is that version. You could put small motors on the bridge with gear drive. Maybe wheels off an HO engine with gears would work.
