onsists in its simplest form of a direct-acting, steam-driven
air-pump, carried on the locomotive, which forces compressed air into
a reservoir, usually placed under the foot-plate of the locomotive.
From this reservoir a pipe is led through the engine cab, where it is
fitted with a three-way cock, to the rear of the locomotive tender,
where it terminates in a flexible hose, on the end of which is a
coupling. The coaches are furnished with a similar pipe, having hose
and coupling at each end, which communicates with one end of a
cylinder containing a piston, to the rod of which the brake-rods and
levers are connected. The application of the brakes is effected by the
engine-driver turning the three-way cock, so that compressed air flows
through the pipe and, acting against one side of the brake-cylinder
piston, applies the brake-shoes to the wheels by the movement of this
piston and the rods and levers connected to it. To release the brakes
the three-way cock is turned to cut off communication between the main
reservoir and the train-pipe, and to open a port permitting the escape
of the compressed air in the train-pipe and brake-cylinders. This
brake was soon found defective and inadequate in many ways. An
appreciable time was required for the air to flow through the pipes
from the locomotive to the car-cylinders, and this time increased
quickly with the length of the trains. Still more objectionable,
however, was the fact that on detached coaches the air-brakes could
not be applied, the result being sometimes serious collisions between
the front and rear portions of the train.

[Illustration: FIG. 1.--Westinghouse Air-Brake.

Section through Triple-Valve and Brake-Cylinder.]

Automatic air-brake.

In the Westinghouse "ordinary" automatic air-brake a main air
reservoir on the engine is kept charged with compressed air at 80 lb.
per sq. in. by means of the steam-pump, which may be controlled by an
automatic governor. On electric railways a pump, driven by an electric
motor, is generally employed; but occasionally, on trains which run
short distances, no pump is carried, the main reservoir being charged
at the terminal points with sufficient compressed air for the journey.
Conveniently placed to the driver's hand is the driver's valve, by
means of which he controls the flow of air from the main reservoir to
the train-pipe, or from the train-pipe