Maths...

Clearly not my strong point...

Having carefully measured the thickness of all six of the driving wheels of my Buckjumper, and produced an average from them.  Then taken various readings of the thickness of my standard back-to-back gauge (and this time scribed the number on the outside face of it to avoid doing it next time) to determine what that needed to be. 

I added them up and started happily cutting down my split axles to length.

I was just cutting into the third and complex one for it has two splits to isolate the gearbox when the penny dropped.  Locomotives have a wheel on each side...

So the axle length should not be 2.46 + 17.78 = 20.24mm

It should be 2.46 + 17.78 + 2.46 = 22.70mm

Oh well, it's a good job that I made up a batch of spares whilst the glue was out!

Cheers
Flymo