On the controlling locomotive, what position does the MU2A or double-port cutout cock need to be in?

The MU2A or double-port cutout cock must be in the 'In' position on the controlling locomotive to ensure that the air brake system functions correctly. When in the 'In' position, the cutout cock allows air flow to the brake system, enabling the brakes to operate as intended. This is crucial for maintaining the integrity and safety of the train's braking system.

By having the cutout cock in this position, the brake pipes and reservoirs are properly connected, allowing for adequate communication between locomotives in a multi-unit setup. If the cutout cock were set to 'Out' or 'Off', it would disrupt the airflow and potentially lead to a failure of the brake system to engage, jeopardizing both the train’s safety and operational efficiency. The 'Closed' position is also not appropriate in this scenario because it indicates a state that would block air flow completely, preventing any brake action from occurring. Thus, the 'In' position ensures full functionality of the air brake system, which is vital for safe train operations.