What must be done when the brake pipe reduction is complete before detaching locomotives?

Closing the angle cock on the locomotive or the cars to be detached is the appropriate action after the brake pipe reduction is complete and before proceeding with detaching. This procedure is crucial for ensuring the integrity of the brake system during the detachment process. By closing the angle cock, you prevent any unintended movement of the detached equipment and maintain the braked status of the remaining train.

This step is vital because it isolates the air brake system of the portion of the train being detached, which safeguards against any accidental release of air and the subsequent uncontrolled movement of the cars left standing. It helps to maintain safety and stability in train operations.

Other options may involve procedures relevant to the overall train setup or maintenance, but they do not address the safety protocol related to the specific action of detachment following a brake pipe reduction. Ensuring that the angle cock on the leading locomotive is closed or that the cars left standing have their angle cocks closed may be standard practices, yet they do not directly pertain to the immediate necessity of the detachment aspect mentioned in the scenario. The same applies for actions relating to handbrakes, which, while important for securing standing equipment, are not part of the protocol following a brake pipe reduction when detachments are made.