They were low cost wheels Doug, in cast iron originally, though they may have moved to cast steel eventually. These machines were built 'cost down' in anticipation of many only having short service lives, behind the front lines.
I am an enthusiast for Bach's WD, not least because it can readily be improved with some simple mods. There's one significant missing detail on the tender, the fire iron rack on the fireman's side. 247 Developments have a brass etch available, but mind your fingers once fitted (probably why Bach left it off). Making a shorter loco-tender drawbar to close the gap, and fitting a fall plate, also helps appearance, and it will still go round curves well below 24" radius.
Then there's the mechanism and performance. It's a bit too light to make full use of the motor and 40:1 gear ratio. The body, once off the chassis, can be dismantled by unscrewing and unclipping the major components. The main ballast weight in the boiler is secured by a screw underneath, and comes out forward once the push-fit smokebox is removed. (The ballast weight looks to me like Bach had thoughts of a smoke unit.) Substitute lead for this ballast and a much more potent hauler results. The chassis is sprung on the second and fourth coupled axles. Reduce the depth of the plastic ribs inside the baseplate to allow some downward travel to these axles, and stretch the springs if adding weight to the loco, this helps both adhesion and pick up.