Today I thought I would talk about air brakes on trains, because I get asked a bit how they work, and trying to explain it makes them sound way more complicated than they really are. Today we will talk about how they came to be, and in a future post, we will talk about exactly how they work.
Air brakes have not always existed on trains. Over a century ago, in the middle of the 1800's, trains were stopped by turning a hand brake wheel on each car. This is where the job of the Brakeman was invented. Typically there would be two brakemen on a train, one on the engine and one on the caboose. When the engineer needed to slow a train down or stop it, he would communicate his intentions using whistle signals. When they brakemen heard the signal indicating that the engineer wanted brakes, the brakemen would go to work, starting at each end of the train and working towards each other. They would walk on the roofs of cars, and turn each brake wheel as they went along. When the train was slowed sufficiently and the brakes needed to be released, the engineer would sound a different whistle signal, and the brakemen would walk across the tops of the cars and release the brakes they had applied. It was a simple system, but it was obviously lacking a few safety features! At one time, a brakeman was considered the deadliest occupation in the United States.
George Westinghouse is the man who invented the railroad air brake. Besides the development of the air brake, he is also well known for his pioneering work in the electrical industry. He recognized that relying on brakemen to stop a train was a very primitive and dangerous system, and he began to work on a system that would allow the engineer to directly control the brakes on the train. Doing so would eliminate for people to be walking on the roofs of train cars while moving, and it would allow the train to stop faster, if the engineer could control all the brakes on the train at the same time.
The very first air brake system was a little different than the air brakes of today. The basic operation of air brakes requires that compressed air be forced against a piston, which in turn transmits that pressure to a brake shoe, generating friction and stopping a train. Early air brake systems worked on that system, known as a straight air system. A pipe ran the length of the train, and on each car there was a brake cylinder. When compressed air was applied to that pipe, it put pressure on each brake cylinder in the train, applying the brakes on every car. When the pressure was released, the brakes released. This offered a huge advancement over the hand operated brakes, but there were still some pretty critical flaws. The biggest flaw with the straight air system is that if cars somehow separate, the pressure is lost in the brake pipe, and brakes release. When cars are not coupled, or a train splits in two, there would be no air brakes available.
A few solutions were offered to fix the problem of losing brakes when uncoupled. In some countries, vacuum brakes were introduced. Vacuum brakes required that a negative pressure be maintained to release brakes. If the brake pipe became separated, due to the train splitting in two, pressure would be restored in the pipe and the brakes would all fully apply. Vacuum brakes had the disadvantages of requiring much larger equipment to generate enough braking force, and being very slow to apply and release. A more popular option was the train air brakes, or automatic brake, which is still in use today on almost every railroad in the world. Automatic brakes work by requiring each car on the train to have a compressed air reservoir, which is filled up by an air compressor on the engine. The brakes are still applied by air pressure acting on the brake cylinder, but that air comes from the reservoir on the car. The brake pipe has constant pressure in it, and a valve ensures that the pressure in the reservoir and the pipe always stay equal. When the engineer needs to make a brake application, he lets some air out of the brake pipe, making the pressure less than that of the reservoir. Air is let out of the reservoir and into the brake cylinder, applying the brakes, until the pressures again reach equilibrium. When the engineer wants to release the brakes, air is added to the brake pipe, making the pressure higher than the reservoir. The brakes release, and the reservoir pressure increases until equilibrium is reached again. Whenever the pressures are at equilibrium, the brakes continue doing what they are doing, but when brake pipe pressure is higher, they release, and when brake pipe pressure is lower, the apply. Next week we will cover in detail exactly how this happens.
As with the other systems, there are advantages and disadvantages to the automatic brake system. One of the biggest advantages is if a train splits in two, or the brake pipe becomes separated, the brakes fully apply and the entire train comes to a stop. There are two major disadvantages, although with proper training one can be eliminated. One disadvantage, which is simply unavoidable, is that having an air reservoir on each car means it takes a considerable time to fully pressurize the brake system. On a long train it can take eight to ten minutes, even on a good day. In extreme cold weather, it can literally take hours. Also, pressure can only be restored to the system when the brakes are released. Consequently, if an application is made, and then released, it takes a minute for the pressure to restore throughout the system. If another application is made before pressure is fully restores, there is less pressure available, and therefore less braking ability available. This is called fanning the brakes, and if done repeatedly it can lead to a complete loss of pressure, and a loss of brakes. With proper training, fanning of the brakes can be avoided, as can any serious consequences of this disadvantage.
Next week we will talk about how exactly the brakes work. How exactly they work is actually considerably simpler than it sounds so far. Basically the thing to remember is that the pressures constantly try to equalize. When the pressure in the brake pipe is higher than the reservoir, then the brakes release. When the brake pipe pressure is lower than the air reservoir, then the brakes apply.
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