Abstract: Each car on a freight train is equipped with what is called a brake control valve. This valve is in the automatic air brake system and causes the air brake on the individual car to be applied when the pressure in the main brake pipe which runs throughout the train is reduced by the engineer or by any other cause. Sometimes, the brake control valve will malfunction, thereby causing it to go into the emergency brake position, either during normal service braking or at other times. When it goes into the emergency position, the brake pipe opens to the atmosphere and the resulting sudden reduction in pressure at that point causes all of the brake control valves on the train to go into emergency position causing a full emergency brake application throughout the train. With long trains, quickly finding the malfunctioning brake control valve is quite a chore.