A wheel to start with. That’s a 20mm  tyre from an old romford wheel soldered to a turned steel disc with a bit of brass tube in the middle. Tube is 3 mm id, 3.5 mm od.

From which a pair of axles The unpowered one has a 3mm silver steel axle , the n20 is a snug fit into the tube for the powered one.  The bearings are Gibson, carefully opened out from 1/8 to 3.5 mm

Bogie frames cut from 1mm brass. I have used the Gibson hornguides. These days I prefer the high level ones for the EM stuff. I find they have a better lateral tolerance, important  for locos with con rods but for this I can use up old stock. There will be some primary suspension, but I will worry about that when I know more about the mass that it has to suspend.

The longitudinal pivot is set back from the centre line, my idea is to put more weight on the driving axles. I don’t know how well that will work yet.

So I tried a quick lash up. A simple brass frame, it is all just sat there for now. Hefty battery as a weight, the bit of string goes to a weight draped over the side of the bench. Rough it looks, but it proves the point, these motors pack a punch for their size. It happily gives 100 g of drawbar pull, jam it solid and the wheels slip rather than locking even with that battery on top. Of course if needed I could always add another pair of traction motors.

Confident that the concept is reasonably practical some further improvements, suspension added and a fuel tank in the centre. The card formers are just wedged in, but allow me to define the internal volume of the engine compartment.

The springs are 0.7 mm pb , they go into a tube soldered to the bogie frame so I can change them later if needs be. Some brass shim on the tops of the wheel bearings so the spring end doesn’t get caught in the groove.

Power and control next.