What key physical characteristic determines the service life of a railway wheel?

The key physical characteristic that determines the service life of a railway wheel is the remaining tread thickness, specifically the distance from the flange (the inner rim of the wheel) to the tread surface (the part that contacts the rail). As a wheel rolls along the rail, the tread surface wears down due to friction and the impact of train loads. This wear reduces the wheel's diameter and alters its profile. The flange is also subject to wear from contact with the rail, particularly on curves. The minimum allowable tread thickness is specified by railway regulations and standards to ensure safe operation. When the tread thickness reaches this minimum limit, the wheel must be re-profiled (worn material is removed to restore the original shape) or replaced. Continued operation with a wheel that has insufficient tread thickness can lead to several problems, including increased stress on the rails, reduced ride quality, and an increased risk of derailment. For example, a wheel with a severely worn tread may not properly engage with the rail, leading to wheel climb (where the wheel tries to ride up and over the rail). The rate of tread wear depends on factors such as axle load, train speed, track conditions, and the type of wheel material. Therefore, regular wheel inspections are essential to monitor tread thickness and identify wheels that need to be re-profiled or replaced before they reach the minimum limit. Once the wheel has been reprofiled multiple times and the condemning limit is reached (due to wheel diameter reaching a minimum value), the wheel must be scrapped.