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 certain quantity of air rushes out of the train-pipe into the small chamber; a further local reduction in the pressure of the train-pipe in that vehicle is thereby effected, and this almost instantaneously actuates the triple-valve of the succeeding vehicle, and so on throughout the train. In this way, on a train 1800 ft. long, consisting of sixty 30-ft. vehicles, the brake-blocks may be applied, with equal force, on the last vehicle about 2½ seconds later than on the first.