Preventing broken wheel derailments and broken rail derailments requires that wheels and rails that are likely to break are identified and taken out of service before they fail. The most common fundamental cause for broken wheels and broken rails is sudden, unstable, dynamic propagation of subsurface fatigue cracks that develop in a stable manner over many millions of cycles of loading. Prior to the final fracture, the cracks are not externally visible. They can only be detected using specialized techniques including ultrasonic inspection. Thus, for example, high impact wheels (HIW), or wheels that deliver dynamic forces to the track structure in excess 80,000 pounds, are not necessarily wheels that are prone to break. This is because the two main causes for HIW — flat spots on the rolling surface of wheels and wheels that are out-of-round — are not well correlated with the presence of the subsurface fatigue cracks that grow and cause the failure.