Thanks for your advice and you have all helped in one way or another. I already have my new two rail track, so don't think switching to three rail is now an option. I also had already sawed through the axels of one of the trucks and cant find any suitably priced Lima wagons, so I thought trying the switch is worth a go. Oscar's video was really useful, as it showed how the wheels were retained, so this is how I went about it. I first made a drill jig from a piece of wood to fit the truck in which I placed a 4mm guide hole 7.5mm in, in line with the rivet. I drilled the rivets out with the 4mm drill, which allowed the chassis to be separated and the couplings and the wheel retainers to be removed. I straightened the ends of the wheel retainers with pliers and put in the new wheels. A bit fiddly but with patience they go in and the retainers can be bent back to hold them in place. Voila I now have new wheels. A couple of washers and a 6BA nut and bolt allowed me to refit the couplings at the correct height. An extra nut and bolt drilled in the centre to hold the body back onto the chassis, with a simulated coal load to cover it all up, gives me a whole new waggon. However, as TH says it can no longer be returned to its original condition, but having seen it run, I am not sure that I want to. Thanks again for giving me the courage to attempt the task.