Train wheels wear just as car tyres do and they need to be checked regularly. When the wear reaches certain limits, the treads either have to be reprofiled to the correct shape or the wheels replaced. Reprofiling wheels is a slow and expensive process but train and wheel design and maintenance has improved considerably over recent years, reducing the periods between visits for reprofiling. Even so, there are still persistent cases of railways running into unforeseen or unusual wheel wear problems and the wheel/rail interface still needs a lot more research before it is fully understood. I captured this scene at a BNSF / Metra maintenance yard in Chicago not to far from union station..