The correct answer is A. 7.8 kg/cm2.
The FP pressure in the brake van of a 40 BCN air braked train must be at least 7.8 kg/cm2 at the starting station. This is to ensure that the brakes are working properly and that the train can stop safely in an emergency.
The FP pressure is the pressure of the air in the brake pipe. The brake pipe is a pipe that runs through the entire train and connects to the brake cylinders on each car. When the brake is applied, air is released from the brake pipe and flows into the brake cylinders. This causes the brake shoes to apply to the wheels, which slows the train down.
The FP pressure must be at least 7.8 kg/cm2 to ensure that the brakes are working properly. If the FP pressure is too low, the brakes may not be able to stop the train in an emergency. This could lead to a derailment or other accident.
The FP pressure is also important for the safety of the train crew. If the FP pressure is too low, the brake shoes may not apply evenly to the wheels. This could cause the train to sway or even derail.
It is important to check the FP pressure regularly and to make sure that it is always at least 7.8 kg/cm2. This will help to ensure the safety of the train and its passengers.
The other options are incorrect because they are not the minimum required FP pressure for a 40 BCN air braked train.