If a flat spot of 4 inches or greater is found, what action must be taken?

When a flat spot of 4 inches or greater is detected on a train wheel, the proper course of action is that the train must remain stopped. This is crucial for safety reasons, as a significant flat spot can lead to increased vibrations, potentially causing damage to the rail, the train, or the infrastructure. This flat spot indicates a loss of wheel integrity, which could compromise the stability and safety during operation. By ensuring that the train remains stopped, it allows for further inspection and repair before the train is allowed to move again, thereby preventing accidents and ensuring the safety of both the crew and passengers.

In contrast, the other options would not be appropriate responses given the severity of the situation involving a large flat spot. Proceeding with caution, notifying the dispatcher, or continuing if other wheels are normal does not adequately address the risks associated with a wheel that has such an excessive flat spot. It is crucial to take immediate and decisive action to halt operations to maintain safety standards.