What procedure must be followed when applying a blue signal to an MU locomotive?

When applying a blue signal to a multiple unit (MU) locomotive, it is essential to pull the emergency brake valve handle. This action is critical to ensure the safety of maintenance personnel working on or near the locomotive. By pulling the emergency brake, the brakes are engaged, preventing any unintended movement of the train while the blue signal is displayed. This helps in safeguarding both workers and equipment during service or inspection.

The other options do not align with the required safety procedures. Shutting down the engine can be inefficient and may not be necessary for the task at hand. Relying solely on a blue tag does not provide the active safety measures needed, and failing to attach any signals would negate the purpose of indicating that workers are present, thus creating an unsafe situation. The correct procedure must prioritize safety and operational guidelines, making the action of pulling the emergency brake valve handle the most effective and appropriate step in this process.