The action is opposite to allow for the emergency application of brakes in case the train air pipe is broken.
Railroad air brakes started with what is called "straight air." This is when the train air pipe is empty until the need to apply the brakes occurs and then compressed air is sent down the pipe from the storage tank on the locomotive. This concept was to be continued for streetcars and for locomotive (only) independent brakes. It is also the concept that was applied to automotive track brakes. But experience soon showed that there was a problem with this concept.
The straight air system required that the air pipe be shut off or otherwise plugged at the end of the train in order that the compressed air from the locomotive would apply the brakes. If this was not done, the compressed air would exhaust and not apply the brakes.
If the train came uncoupled (break-in-two) there was no way to apply the brakes on the remaining front section of the train because the compressed air would exhaust from the open end of the train line. And other than using the hand brakes there would be no way to stop the rear section of the train.
So George Westinghouse very soon developed the concept that we know call automatic air brake. This method keeps the trainline pipe full of compressed air at all times. At each car a storage reservoir on the car is thus charged with the compressed air. Each car has a pneumatic (air) relay valve that senses the pressure in the trainline. When the pressure in the trainline is reduced this valve, whether it is a K-style or an AB (or modern variant), responds by sending air from the storage tank to the brake cylinder. The amount of air going to the cylinder is proportional to the reduction of pressure in the trainline. If the trainline because discontinuous due to an uncoupling or other incident. the air in the pipe is exhausted and the brake value moves to its extreme and essentially connects the full pressure of air in the storage tank to the cylinder.
Similarly a conductor could operate a "conductor's value" in the caboose or within a passenger car and cause the air brakes to be applied if he felt that the engineer was unaware of a problem behind the locomotive.
The action is opposite to allow for the emergency application of brakes.
Brian Norden
P.S. Some streetcar systems would operate a motor car and a trailer car and use straight air to operate the brakes in both cars together. The Pacific Electric would operate up to three car trains of its suburban cars using straight air braking. These cars were often equipped with an emergency valve that worked similar to a triple valve so that if a conductor's valve was operated the air in the on-car storage tank was connected to the brake cylinder.
