compressed air in the brake-cylinder is discharged, and the piston is
brought back by the coiled spring, thus releasing the brakes. At the
same time the auxiliary reservoir is recharged.

Quick-acting air-brake.

With this "ordinary" brake, since an appreciable time is required for
the reduction of pressure to travel along the train-pipe from the
engine, the brakes are applied sensibly sooner at the front than at
the end of the train, and with long trains this difference in the time
of application becomes a matter of importance. The "quick-acting"
brake was introduced to remedy this defect. For it the triple valve is
provided with a supplementary mechanism, which, when the air pressure
in the train-pipe is suddenly or violently reduced, opens a passage
whereby air from the train-pipe is permitted to enter the
brake-cylinder directly. The result is twofold: not only is the
pressure from the auxiliary reservoir acting in the brake-cylinder
reinforced by the pressure in the train-pipe, but the pressure in the
train-pipe is reduced locally in every vehicle in extremely rapid
succession instead of at the engine only, and in consequence all the
brakes are applied almost simultaneously throughout the train. The
same effect is produced should the train break in two, or a hose or
any part of the train-pipe burst; but during ordinary or "service"
stops the triple-valve acts exactly as in the ordinary brake, the
quick-acting portion, that is, the vertical piston and valve seen in
fig. 1, not coming into operation. When the handle Z is turned to the
position X the quick-acting mechanism is rendered inoperative, and
when it is at Y the brake on the vehicle concerned is wholly cut out
of action.

A further improvement introduced in the Westinghouse brake in 1906 was
designed to give quick action for service as well as emergency stops.
In this the triple-valve is substantially the same as in the ordinary
brake. The additional mechanism of the quick-acting portion is
dispensed with, but instead, a small chamber, normally containing air
at atmospheric pressure, is provided on each vehicle, and is so
arranged that it is put into communication with the train-pipe by the
first movement of the triple-valve. As soon, therefore, as the driver,
by lowering the pressure in the train-pipe, causes the triple-valve in
the foremost vehicle of the train to operate, a cert