The function of the engineer's valve is to control these operations. Naturally the runner's left hand rests on this instrument, which is fixed to the back head of the boiler. To apply the brakes he turns the handle to such a position as to allow air to escape from the train-pipe; to release, he turns it to allow air to pass from the main or locomotive reservoir into the train-pipe, and thence into the car-reservoir. It is hardly necessary to say that the operation of the brake, which has been described for one car, is practically simultaneous throughout the train. The brakes on the driving-wheels of the engine are also automatically applied at the same time as those of the cars and the tender.
In the plan on [page 197] the several different positions of the handle of the engineer's valve are indicated, and among them the service-stop and the emergency-stop positions. The quickness of the stop can be to some degree controlled by the rapidity with which the air-pressure in the train-pipe is reduced. To make a stop in the shortest possible time, the runner moves the throttle lever with his right hand and shuts off steam, and with his left hand moves the handle of the engineer's valve to the emergency position, then pulls the sand-rod handle to let sand down to the rails, and finally, if the engine is not fitted with driver-brakes, he must reverse the engine and again open the throttle. These movements must be made in order and with precision; and to make them instantly and without mistake in the face of sudden danger requires coolness and presence of mind. It sometimes happens that an engine-runner reverses his engine before shutting off steam, in which case the cylinder-heads will very likely be blown out and the engine be instantly disabled. Then, if there are no driver-brakes, the locomotive is worse than useless, for instead of aiding in making the stop, its momentum adds to the work to be done by the train-brakes. Again, if the air-pressure in the brake-cylinders is so high, and the adjustment of the levers such that an instant application of the full pressure will stop the rotation of the wheels, and cause them to slide on the rails, the stop will take longer than if the wheels continued to revolve. The maximum braking effect is obtained when the pressure on the wheels is as great as it can be without causing them to slide, and it may happen that a quicker stop can be made by putting the engineer's valve to the service-stop position than by trying to make an emergency-stop. The runner must, therefore, be familiar with the special conditions of his brakes, and must have that kind of mind which can be depended upon to work clearly and quickly in a moment of tremendous responsibility. Fortunately, such minds are not very rare. The world is full of heroes who want only discipline, habit, and opportunity.
The pressure of air in the main reservoir and the train-pipe is maintained by the air-pump on the locomotive, the speed of which is automatically regulated by an ingenious governor. It is the throbbing of this vigilant machine which one hears during short stops at stations. The air-pressure has been reduced in applying the brakes, and the governor has set the pump at work.
All of those parts of the air-brake apparatus which are shown in the diagram ([pp. 196–97]) can be easily seen on a train standing at a station; but the curious traveller must be careful not to mistake the gas-tank carried under some cars for the car-reservoir. The gas-tank is about eight feet long; the car-reservoir is about thirty-three inches.
Although the air-brake can almost talk, it is still not perfect. There are several fortunes to be made yet in improving it. For instance, it is desirable, in descending long and steep grades, that the brake-pressure should be just sufficient to control the speed of the train, and should be steadily applied; otherwise the descent will be by a succession of jerks which may become dangerous. With the automatic the brakes must be occasionally released to recharge the reservoirs, or when the speed of the train is too much reduced; and it is difficult to keep a uniform speed. So far, the means devised to overcome this difficulty and keep a constant and light pressure on the wheels have been thought too costly or complicated for general use. With hand-brakes long trains are controlled by the brakes of but a few of the cars in any one train. It follows that in the descent of grades the braked wheels must often run for miles with the pressure as great as it can be without sliding the wheels. The rim of the wheel is rapidly heated by the friction of the brake-shoe, and the unequal expansion of the heated and the unheated parts of the wheel causes a fracture. This is why so many broken car-wheels are found at the foot of grades—of all places the worst for such an accident to happen. With "straight air," that is, with the pressure from the main reservoir, or the air-pump, going directly to the brake-cylinder, the engineer can apply the brakes to all the wheels of his train simultaneously, and with great delicacy of graduation; and by turning a three-way cock which is placed in the piping of each car, the air can be used "straight." This is regularly done on some mountain-roads. At summits the trains are stopped and the brakes are changed from "automatic" to "straight." This practice is dangerous, however, and is not approved by the best brake-experts, for if a hose bursts, or through some other accident the air in the train-pipe escapes, the brakes are useless. The automatic arrangement by which a reduction of air-pressure in the train-pipe applies the brakes, as previously explained, is much preferred, although no entirely satisfactory means has yet been devised for automatically regulating the air-pressure in the brake-cylinder.
There is not space here to enter into the history of the air-brake. It was first practically applied to passenger trains in 1868. The first great epoch in its subsequent development was the invention, by Mr. George Westinghouse, Jr., of the triple valve. The introduction of the triple valve at once reduced the time of full application of the brake for a ten-car train from twenty-five seconds to about eight seconds. This means, at forty miles an hour, a reduction by more than one thousand feet in the distance in which a train can be stopped. The next great epoch in the history of the air-brake was made by the celebrated Burlington brake-trials of 1886 and 1887. These trials were undertaken by a committee of the Master Car-builders' Association, to determine whether or not there was any power-brake fit for freight service. For general freight service the brake must be capable of arresting a very long train, with cars loosely coupled, running at a fair average passenger speed, without producing objectionable shocks in the rear of the train. The two series of trials were carried out in July, 1886, and May, 1887. The competing brake-companies brought to the trials trains of fifty cars each, equipped with their devices. Skilled mechanical engineers from various railroad and private companies assisted both years. These trials were most exhaustive, and have contributed more to the art of braking than any that preceded or have followed them. The first year's trials developed the fact that the air-brakes could not be applied on the rear of a fifty-car train in less than eighteen seconds, whereas the head of a train moving twenty miles an hour could be completely stopped in fifteen seconds. The result was that disastrous collisions between the cars of any one train were produced in the act of stopping. Men in the rear cars were thrown down and injured, and much damage was done to the cars. At the end of nineteen days the brake-companies went home to work another year over the new problem. In 1887 they reappeared on the same ground, and in eighteen days proved that no simple air-brakes, as then operated, could prevent disastrous shocks in a long train; but it was shown that by bringing in electricity to actuate the air-valves, the application of the brakes could be made practically simultaneous throughout the train. Mr. Westinghouse, however, during the summer following, made such modifications in the triple valve and in the train-pipe that he succeeded in applying the brakes throughout a fifty-car train in two seconds. That settled the matter. He at once equipped a train of fifty cars, and in October and November, 1887, that train made a journey of about three thousand miles, making exhibition stops at various cities. The journey was a splendid and conclusive demonstration that the air-brake is now a thoroughly efficient and reliable contrivance for freight as well as for passenger service. The result has been a very rapid application of the new quick-acting brake to freight cars. The performance of this train was to railroad men most impressive. A freight train of fifty cars is about one-third of a mile long. To see such a train, running forty miles an hour, smoothly stopped in one-third of its own length, without shock or fuss, was an object-lesson that no one could fail to understand or to remember. Some of the stops made by this train will give a fair notion of the relative power of hand- and air-brakes for quick stops. The following figures are averages of stops made in six different cities. They give the distances run in feet from the instant of applying the brakes till the train was brought to a stand-still:
| Feet. | |
| Hand-brakes, 50 cars, 20 miles an hour | 794 |
| Air-brakes, 50 cars, 20 miles an hour | 166 |
| Air-brakes, 50 cars, 40 miles an hour | 581 |
| Air-brakes, 20 cars, 20 miles an hour | 99 |
With twenty cars at twenty miles an hour even shorter stops were made than those recorded above. In the Burlington trials the hand-brake stops, with fifty-car trains at forty miles an hour, were made in from two thousand five hundred to three thousand feet.
