Final answer:
If a lock is found to be defective or missing on a main line switch or derail, the employee must report the issue and refrain from using the equipment until it is repaired or replaced.
Step-by-step explanation:
If a lock is found to be defective or missing on a main line switch or derail, it is crucial for an employee to take immediate action to prevent any accidents or injuries. They should report the issue to their supervisor or safety officer and refrain from using the equipment until it has been properly repaired or replaced. By following these steps, the employee ensures the safety of themselves and others.