All the wheels and axles were looking a bit rusty. I think I'd missed some flux cleaning on the chassis, and fumes in the closed box has affected the surface. A hour with mild abrasive, a fibre pen and even Brasso saw them restored to good condition, at which point I noticed something odd.
Each axle has a worm on the end, for the wheel to screw on to. The other is plain for an insulating plastic bush to slide on.
I've never seen this before, and can't think of any good reason for it. Surely, some of the time, the wheel is rotating in a way that will unscrew it from the axle? I suppose a shunter is likely to travel as far backward as forward, but I just see the wheel coming off at some point, or at least unscrewing enough to make trackholding tricky.
Can anyone explain? I'm confused.