What should be done if a flat spot exceeds the length of 2 inches but is less than 4 inches?

When a flat spot on a wheel exceeds a length of 2 inches but is less than 4 inches, it is important to limit the speed of the train to ensure safety and minimize potential damage. The requirement to not exceed a speed of 10 mph is a precautionary measure. This limitation reduces the risk of further wear on the wheel and helps maintain control over the train while passing over the affected spot. Operating at reduced speeds allows for a safer journey and provides the opportunity to address the issue more effectively, such as planning for wheel replacement or maintenance when it is safe to do so.

In contrast, continuing at normal speed or taking no action can lead to increased risk of derailment or additional damage to the train. Furthermore, requiring the engineer to replace the wheel immediately may not be practical, especially if the train is still in service and can be safely operated at a reduced speed.