 If the flat spot is very large (as in the picture), strands of molten metal may have stuck on one side of the flat spot, making it impossible for the wheel to turn due to insufficient clearance between the rolling surface and the brake block.In this case, the wheelset must be replaced immediately.In extreme cases such as the 1971 Salem, Illinois derailment a wheel with an untreated flat spot can damage the track and cause a derailment.