(2) Functioning of Air Brake Systems. Air brake systems operate through storage
reservoirs that are located on each car and are charged with compressed air. These reservoirs are
separated from the brake pipe which delivers the compressed air by a brake valve. The brake
valve senses pressure drops in the brake pipe and utilizes compressed air from the reservoir to
apply the brakes. Since a pressure drop initiates brake applications, the design has a built-in
break-in-two safety feature. A significant quick loss of air, such as would occur in a
break-in-two, will result in an automatic emergency (full force) application of the brakes using air
located in a separate emergency reservoir.
The air brakes are controlled from the control stand in the locomotive. The operator
moves the automatic brake valve to release air in the brake pipe. This loss in air pressure is
sensed by the individual brake valves on each railcar in the consist. Each railcar brake valve then
applies air pressure (in direct proportion to the brake pipe reduction caused by the operator) from
the air reservoir located on the railcar to the brake piston thus applying the brakes on the railcar.
To release the brakes, the operator restores pressure to the brake pipe by placing the automatic
brake valve in the release position. The brake valves on each railcar in the consist sense the
increasing pressure and exhaust the brake piston pressure and thus release the brakes. Brake pipe
pressure is then used to recharge the air reservoir on the railcar. After a period of time
equilibrium will again be reached, the brake pipe will be at its feed cock pressure, all brakes will
have released, and all railcar air reservoirs will be fully recharged.
(3) Types of Brake Systems. Railroad cars found on Naval Activities have a variety of
air brake systems ranging from the K type brake on older cars to ABDW types of braking systems
on the newest cars with AB and ABD systems being the most common.
(a) K Brake Systems. The K series of brake valves, KC for a brake cylinder
combined with the air reservoir and KD for the brake cylinder being detached from the air
reservoir, came into use in the early 1900s. Only the oldest of the Navy's cars have this type of
system and they are being phased out. See Figure 2-9 for a picture of a KC brake valve and air
(b) AB Brake Systems. The AB brake provided the following advances over the
K type: slight brake pipe pressure reductions do not cause unintended brake applications; quick
service propagation is twice as fast; releases and recharges are faster; brake cylinder pressures are
more even; and there is less sensitivity to brake pipe leakage. On AB brake systems, the brake
valve, brake cylinder, and air reservoir are mounted separately. See Figure 2-10 for an illustration
of AB brake valve.
(c) ABD Brake Systems (1961). The main advancement over the AB valve is the
use of diaphragms and 0-rings in place of pistons which reduce wear, weight and repair
complexity. See Figure 2-10 for an illustration of an ABD brake valve.
(d) ABDW Brake Systems (1976). This is the current brake system installed on all
new cars. Its main advancements over the ABD are faster brake applications and continuous,
quick action through an accelerated service application valve. See Figure 2-10 for an illustration
of an ABDW brake valve.