Air-Brake, in railroading, a mechanical contrivance by which compressed air is used to stop railroad trains or to regulate their speed. In its simplest form an air-brake consists of four parts, a condensing air-pump, a train-pipe, a brake-cylinder, and a brake. The pump, which is located on the locomotive and is under control of the engineer, stores compressed air in a reservoir. The train-pipe runs back under the train. It is composed of car lengths coupled together to form one continuous pipe. The brake-cylinders, one under each car, are supplied with compressed air through the train-pipe. The piston of each brake-cylinder works a lever which, in turn, forces the brake-shoe against the wheel of the car. When the engineer desires to retard the motion of his train, he merely throws a lever and permits air to escape from the reservoir into the brake-cylinders, thus forcing the pistons and the brake-shoes to do their work. This simple form, known as the straight air-brake, was placed on trains by Mr. George Westinghouse in 1869. Mr. Westinghouse has devised numerous improvements. The different forms are known as the straight air-brake, the automatic air-brake, the quick action air-brake, and the high speed air-brake. The high speed form was devised about 1891. With the latest and most approved equipment an engineer can bring a heavy train running at full speed to a dead standstill within an incredibly short distance-"almost within the throw of a hat," as one writer puts it. The common car-brake consists of an upright rod which may be turned by means of a wheel or crank. As the rod turns, it winds up a chain which draws the crank-shoe against the car wheel. If the engineer of a train equipped with the old brake desires to slow up or to stop, he whistles "down brakes." The brakeman makes all speed to twist up his brakes. In the case of a freight train, it is necessary to run along the tops of the cars to reach the brakes. Ice, darkness, and overhead bridges render the service full of danger. There is still need of the ordinary brake on each car as a means of control when the car is cut out of the train, but there is a gratifying tendency in modern legislation to require that all trains, both freight and passenger, be provided with air-brakes.