With the sides and floor fitted, I hit another problem - the motor and gearbox wouldn't go in. Although it fits fine without filling the cab, to actually insert the thing, the 'box does have to take up space that it subsequently relinquishes again.
So, on the basis that this is supposed to be a working model, I had to make space elsewhere. The obvious thing to do is trim away the back end of the boiler. Initially I used small files but quickly realised that I needed something bigger and moved on to the minidrill and slitting disk. The motor used is quite tubby the hole is wider than I'd like but once on the track you won't see it.