Given that this is the first locomotive I've built where I've had to form the entire chassis and gearbox I'm really happy to see all the wheels turning move under their own power. The test did highlight a slight issue with the way I'd put the gearbox together though.
If you look at the gear on the rear axle you will notice that on the lower surface it has a boss that extends out along the axle which isn't present on the top surface (you can just about see this in the photo to the left, but it is a lot clearer in the photos from the previous post. The problem is that when the gearbox is fitted the larger part of the gearwheel is fouled by the boss if it is fitted on that side of the axle gear.
Fortunately it is easy to fix this by simple rotating the gear in the gearbox (given I can't swap the wheels around as they are permanently trapped in the chassis). Although the worm and gear are now no longer in line they still mesh nicely and everything turns freely.