Why is it necessary to ensure all angle cocks are left open on standing portions of a train?

The necessity of leaving all angle cocks open on the standing portions of a train centers on the importance of maintaining brake pressure throughout the train system. Angle cocks are valves that control the flow of air in the braking system. When these cocks are left open, it allows for an uninterrupted air supply to the brake cylinders, which helps maintain adequate brake pressure. This is crucial in ensuring that the brakes remain effective and ready to engage when needed, especially in case of an unexpected movement or emergency scenario.

By keeping the angle cocks open, the consistent air pressure ensures that the brake system remains in a functional state, thus enhancing safety for both the train and its surrounding environment. If the angle cocks were closed, it could lead to a drop in brake pressure, potentially making the train less secure and increasing the risk of unintended movement.