Which of the following actions is prohibited when a blue flag is displayed?

When a blue flag is displayed, it serves as a signal that indicates an area is protected from incoming trains or equipment movement, signifying that work is being performed or that personnel are present in that area. Actions that could compromise this safety, such as adding to train or equipment, are explicitly prohibited. This means no trains, equipment, or additional cars can be brought into that work zone, which could potentially endanger those working there or disrupt the safe environment that the blue flag establishes.

Conducting routine checks, charging batteries, or taking a break for maintenance are all actions that can typically occur within the parameters of a blue flag display, provided they do not interfere with the protection that the flag is meant to provide. Therefore, the prohibition against adding to train or equipment is correctly identified as the action that violates the safety protocol represented by the blue flag.