It turns out that the cheap Chinese motors I bought on eBay are just that: cheap. Despite much fiddling with the motor position I couldn't get the original motor to turn the wheels while on the track. The chassis rolled freely with all of the gears and rods attached, but the motor would only turn the wheels if I held the chassis in mid-air, and then only if I held the motor in exactly the right place. On the track, it just wouldn't budge. Eventually I came to the conclusion that the motor just didn't have enough torque, and bought one of the Tramfabriek 0716 motors which are (newly) available from the 2mm Scale Association shop. The difference was magical!
Here you see the new motor test fitted. The worm is now brass, because I couldn't get the old plastic one off the motor shaft. The eagle-eyed will spot that I replaced the worm bearing bracket with a heftier version, filed up from a block of solid brass rather than a bit of thin brass angle. This was part of my messing around with the eBay motor - I thought it was too loose and that might be preventing the worm from meshing correctly. Again, it has a phosphor bronze frame bush soldered in to take the little length of stub axle, which is a press fit into the worm. The worm sits on a 1mm shaft adaptor like before (I think it came from Nigel Lawton). This motor has a double-ended shaft, as you can see. Unfortunately I can't take advantage of this and fit a flywheel as there isn't room.
Here is the motor after cutting off the unused shaft. This was done with a slitting disc in the minidrill. I had to cut it off quite close to the end of the motor because there is very little clearance lengthwise in the bunker. Even so, I had to file a bit off the motor support collar to get the body on. I found it useful to attach the wires to the motor with Kapton tape.
At long last, the chassis has made its first successful test run - here's a video. For this I temporarily soldered the motor wires to the side frames, and balanced some lumps of lead on top to give it some hope of current collection. It's a bit more hesitant in the backward direction, but I will worry about this once I've dismantled the chassis for painting and put it back together again!