Unbelievable! Very convincing model. I agree with thecolclough and lynn22. The hubs of the wheels can remain blue (excellent texture btw) but the parts that contact the rails (the rim and the outer face of the wheel flange) should be silver, even quite polished, from the constant friction. Great work on the details, like the chain and the suspension setup. Looks like a volume shadow error on the right but no big deal.
I am no expert on rolling stock, but shouldn't it have a coupling mechanism to allow it to attach to a locomotive? I would guess the chain would be a part of that mechanism, as a safety backup in case of coupling failure, but the main load would be taken by a hook-like arrangement. But like I said, I don't know much about trains and stuff. Excellent work, it will look great in the project!