Having said that the next post would be about the coupling rods, this will have to wait as I have been working instead on mounting the motor. I'm using a 7mm diameter coreless one from eBay, which has no built-in way of fixing it in place. So after some pondering I decided to make a collar that can be tightened around the motor to hold it. This started as a length of scrap 0.25mm nickel silver from the sprue of an etched kit. It was easily formed into a circle using a brass bar in the vice with the jaws open:

I bent out two "legs", and drilled a hole to 14BA clearance through them, as in the next photo. This will allow the collar to be tightened on to the motor with a nut and bolt:

I also soldered a 12BA nut onto the collar at 90-degrees to the tightening screw (which will go in the coal bunker). This 12BA nut allows the collar to be screwed onto the chassis block, through a hole drilled for the purpose. In the photo below, you can see the collar in place. I've fitted a washer under the 12BA nut in an (ongoing) attempt to find the best height for meshing the worm. It would probably have been better to hold the motor at the business end (in part to avoid having it in the rear of the cab), but there wasn't space to attach it there owing to the hole for the chassis fixing screw.

Here is an underneath view, showing the 12BA screw - I recessed the head in the chassis block. Note that the 14BA nut is soldered onto one side of the collar leg, so that you can adjust the screw holding the motor in place without having to hold the nut in position.

Here the plastic 30:1 worm has been fitted. Since it has a wider bore than the motor shaft I used one of the Association shaft adapters, on to which the worm is a tight push fit. The adaptor was a loose sliding fit on the shaft of this particular motor, so I secured it with Araldite. There was a danger of gumming up the works in this operation, so I used a temporary barrier of cigarette paper and additionally kept the motor turning while the glue dried.

Trials now indicated that the worm needed to be supported at the other end in order to remain properly engaged with its wheel, so I cut a piece of brass angle and attached a frame bush:

This simply screws into a tapped 12BA hole in the chassis block:

The nice things about this arrangement are that (1) all of the screws can be tweaked to get it to run just right, and (2) the motor could be replaced if need be. It and the worm can just be slid out backward. Getting it to run is not as easy as it appears - in fact, I find it works best currently with at least one screw slackened off, which probably indicates that something is not lined up somewhere, but I haven't been able to find exactly where yet. Below you see me running in the gears with the motor temporarily powered up. As you can see, I didn't end up attaching the worm to either of the side frames, for which I provided the tall extensions. The current plan is to leave these in place as extra weight, unless I find that the space is needed for something else!