Wheels are usually based on car count. My guess is I get about 30K cars out of a set. This wheel shaft likely wore out because the bearing collar was not firmly attached, allowing the shaft to spin inside the bearing instead of the bearing itself rotating. You should check the wheels a couple of days after installation to make sure the set screws on the collars are still tight. I think its a good idea to change drive wheels in pairs but as Pat said you can change the idler wheel if the diameter is not too different from the other wheels. I measure the circumference with a flexible tape measure and compare to new. As I get older and want to screw with these machines less I'm starting to consider changing everything: wheels, bearings, motors, gearboxes, every 25K cars and be done with it for a couple of Years. If I wait for a problem to arise this stuff inevitably fails at the most inconvenient time.
