The railbus is only powered on a single axle - this should be more than sufficient for a tiny rail vehicle that will never pull anything. The gearbox folds up and is soldered to the chassis plate.
A little open frame motor screws into place and then wheels ans gears are fitted. The last owner strengthened some of the bends with fillets of solder, not a bad idea as the brass is pretty thin.
The gears don't have grub screws to retain them. On the axle a dollop of superglue does the job (Loctite would be more engineeringly but I don't get on with it) and the worm pushes on to the motor shaft without fixative.
The result seems to work OK., Top speed doesn't seem too mad but I'll get a better idea once the other wheels are in.