The correct answer is C. 5 kg/cm2.
The air brake system on a train uses compressed air to apply the brakes. The air pressure in the system is regulated by the brake valve, which is located in the engine. The brake valve is set to a specific pressure, and when the train is in motion, the air pressure in the system is maintained at that level. When the brakes are applied, the brake valve releases air from the system, which causes the brakes to engage.
The amount of air pressure in the system is important for the proper operation of the brakes. If the air pressure is too low, the brakes will not be able to stop the train effectively. If the air pressure is too high, the brakes may lock up, which can cause the train to skid.
The recommended air pressure in the system for a 40 BCN air braked train is 5 kg/cm2. This is the minimum pressure required for the brakes to operate effectively. If the air pressure drops below this level, the brakes may not be able to stop the train in an emergency.
It is important to check the air pressure in the system regularly and to make sure that it is always at the recommended level. This will help to ensure that the brakes are always in good working order and that the train can be stopped safely in an emergency.
The other options are incorrect because they do not represent the recommended air pressure in the system for a 40 BCN air braked train.