Most freight train wheelsets are a scale 33 inches in diameter. As Pete said, you will need to buy replacements to match the trucks. I've bought Atlas low profile wheelsets for code 55 N scale track for Atlas and Micro Trains. Wheels for Atlas trucks will not work with MT trucks and MT wheels will not work with Atlas trucks do to the different lengths. All I've replaced have been plastic so not much help there. But there are different lengths among manufacturers so be aware.
"N", the Normal scale!