I had to reduce the length of the driving axle by 18mm so that it would fit between the frames of the railcar. It was only after cutting 9mm off of each end of the axle that I noticed the gear wheel was not central to the axle. As it had been forced on to a splined part of the axle to prevent slipping, trying to centre it did not seem to be an option. However a spare gear is provided with the gearbox and I happened to have a 1/8 inch diameter axle of the right length in stock. I was therefore able to make up a new axle and gear.
I had to cut away a substantial part of the upper section of the gearbox frame to allow me to fit the gearbox and motor under the floor of the railcar. I did not need that part of the assembly, it's there for locating a fixing pin when the unit is fitted between the frames of a loco. My motor unit is held in place by a strip of brass shaped to form a U bracket.
I have re-assembled the railcar and it runs better than it ever did with the old motor and gearbox, running smoothly in both directions. Something it only did in one direction before.