On the 3-window, wood cabooses, the Frisco used a combination whistle and airbrake valve, which was mounted on the end railings of each platform. The whistle was used during back-up moves, and the air brake valve was used to dump the air during an emergency. The first image depicts the air line valve closed which is the normal position. The small button, which is near the bottom is the whistle valve. Pushing it will blow the whistle. The second view is a side view, and it shows the port through which the train line air exhausted when the brake valve was opened. The third view, shows the brake valve completely open or in emergency. I will have to hook up the whistle to my air compressor, and see if I can make a decent recording.