Cars or engines protected by blue signals must not be what?

The correct response highlights a critical safety protocol in railroad operations concerning blue signals. When cars or engines are protected by blue signals, it indicates that they are in a state where workers are present or performing tasks that require those units to remain stationary. This protection is essential to ensure the safety of the employees working in that area.

Movement or coupling to these protected cars or engines would pose a significant danger, as the personnel might be unaware that the units are being moved. This could lead to accidents or injuries, undermining the very purpose of the blue signal, which is to create a safe working environment.

Understanding this protocol is crucial for maintaining safety in railroad operations and ensuring that all personnel are kept safe from unexpected movements of rolling stock. Safety is prioritized in all operations, and the blue signal acts as a vital visual tool for signaling that certain procedures are underway that must not be interrupted by unauthorized movements.