It's turning out to be a bit more complicated, which I would expect from Rapido, after reading the exploded diagrams.
The locomotive wheels are all mounted on shouldered axles and are insulated both sides using plastic bushes - similar to the Bachmann spec, and also IRM diesels so far. Shouldered axles have some advantages in that they are good at preserving the back-to-back measurement - once you hit the stop, that's it.
This means having P4 shouldered axles specially made. Possibly Ultrascale can do this, as they already make them for their GWR 44xx driving wheels, albeit a ⅛" dia. axle turned down to 2.5mm dia. at the ends. Rapido's are a different diameter on both counts.
I also must measure the overall width of the driving wheels in particular and see if it is practical to have metal skimmed off the back to thin the flanges.
The tender wheels are very familiar if you have encountered IRM coach bogie wheels - disc wheels mounted on a 1.5mm stub axle, and kept to gauge by a central insulating tube. There are ways of moving the wheels out with press tools.