[livesteam5629]

Take a look at Sierra Valley wheels or any metal wheel sets for that matter. The have a nylon bushing around the axel. Why not press off the wheel from the axel, enlarge the axel hole and press in a nylon or other insulating material bushing, then put the wheelset back together. Only problem with this is keeping a path for the electricity isolated. If the wheel or tire rubs the frame you lost the insulation. Another way without having to machine a new tire is to turn down the wheel center, wrap insulating material around it and press the tire back on.
Just my thoughts.