The undercuts in the wheel flanges are a sign of wear on the wheel flange.

Note

First check the clearance between wheel flange and rail. If the clearance between wheel flange and rail is too large, the wheel flanges must be inspected.

   Inspect the wheel flanges of the wheels: The wheel flanges must not be worn so much that a continuous crack can be seen in the wheel flange (rather than undercuts). If they are, replace the wheels. See Disassembling the powered wheel.