After cars have been added to a train, why must the brakes be applied in emergency from the added cars?

When cars are added to a train, applying the brakes in an emergency from the added cars is crucial to ensure that the angle cocks are open throughout the train. Angle cocks are valves located on the air brake lines that are used to control the flow of air for braking. When new cars are added, especially if they have not been properly checked, it's important to verify that these valves are functioning correctly and are fully opened. If the angle cocks are closed or not functioning, it can disrupt the brake system, leading to unbalanced braking and potentially creating hazardous situations.

Using the emergency brake application allows the operator to ensure that all brakes throughout the train, including those on the newly added cars, engage uniformly. This is a critical safety measure to prevent issues such as rolling or uneven braking force, which could potentially derail the train or lead to other dangerous conditions. Ensuring that the angle cocks are open and the brakes work correctly helps maintain the overall integrity and safety of the train operation.