The picture (above) shows our 20mm Quad' installed in the Judith Edge kit for which it was designed, with the unit spanning between the two inner-most axles of the bogies.
 
These short wheelbase power units would also be suitable for use in small shunters, and you can choose one of the three available ratios - ranging from slow, to very slow - to suit the diameter of your wheels.
 
The QuadDriver 20mm sits directly on your loco axles with the weight of the engine supported by your own sideframes or chassis. This design allows you to arrange one of the driven axles so it can rock as part of a simple 3-point compensation system.

The units are small and very compact with an final drive gear of only 8mm diameter. The inter-linking geartrain is tucked right up, running just under the motor, so it doesn't show under the loco.