Hi Neil
You said: I would take it from that that the Rivarossi Big Boys don't have RP25 wheels then. I have the Trix Big Boy which does have RP25 wheels as it was intended for the US market. It's interesting that some manufacturers offer this option and some don't.
*** The Rivarossi Big boy has been produced under Rivarossi, AHM and I think for a while Walthers brands - the same loco has been going in one way or another for for perhaps 30 years.
Initial production had those larger flanges, then the became a little smaller, then RP25 or close to it over the last few years of Rivarossi's existence.
Alistair: The biggest thing in your initial comment was the death of the gearboxes... almost certainly, this is a good example that all modellers should be aware of - This is a very uncommon fault.
the "disintegration" is almost certainly due to an earlier owners use of the wrong lubricant, with the chemicals attacking the plastic.
If the other loco came in the same purchase, just in case, I'd strip it and clean it thoroughly with a good stong detergent than reassemble with a good "safe" lubricant!