Final answer:
The air pressure in the train air brake system will be reduced to zero if all the air is released from the system.
Step-by-step explanation:
The air pressure in the train air brake system will be reduced to zero if all the air is released from the system.
When the air is released from the system, the pressure inside the brake system decreases and becomes equal to the atmospheric pressure outside. This causes the brakes to release and the air pressure in the system to reach zero.
One example of this is when the train's engineer applies the brakes, the compressed air is released from the system, reducing the air pressure to zero and allowing the brakes to disengage.