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From Wikipedia: 'The automatic vacuum brake consists of a continuous pipe -- the train pipe -- running throughout the length of the train. In normal running a partial vacuum is maintained in the train pipe, and the brakes are released. When air is admitted to the train pipe, the air pressure acts against pistons in cylinders in each vehicle. A vacuum is sustained on the other face of the pistons, so that a net force is applied. A mechanical linkage transmits this force to brake shoes which act by friction on the treads of the wheels. The brake cylinder is contained in a larger housing. This gives a reserve of vacuum as the piston operates. The piston in the brake cylinder has a flexible piston ring that allows air to pass from the upper part of the cylinder to the lower part if necessary. 'When the vehicles have been at rest, so that the brake is not charged, the brake pistons will have dropped to their lower position in the absence of a pressure differential (as air will have leaked slowly into the upper part of the cylinder, destroying the vacuum). 'When a locomotive is coupled to the vehicles, the driver moves his brake control to the "release" position and air is exhausted from the train pipe, creating a partial vacuum. Air in the upper part of the brake cylinders is also exhausted from the train pipe, through a non-return valve. If the driver now moves his control to the'brake' position, air is admitted to the train pipe. According to the driver's manipulation of the control, some or all of the vacuum will be destroyed in the process. The ball valve closes and there is a higher air pressure under the brake piston than above, and the pressure differential forces the piston upwards, applying the brakes. The driver can control the severity of the braking effort by admitting more or less air to the train pipe. 'Release valves are provided on the brake cylinders; when operated, usually by manually pulling a cord near the cylinder, air is admitted to the upper part of the brake cylinder on that vehicle. This is necessary to release the brake on a vehicle that has been uncoupled from a train and now requires to be moved without having a brake connection to another locomotive, for example if it is to be shunted. 'Today's largest operators of trains equipped with vacuum brakes are the railways of India and South Africa. Other African railways [also] use the vacuum brake. Other operators of vacuum brakes are narrow gauge railways in Central Europe, the largest of which is the Rhaetian Railway [in Switzerland, which features adhesion working over grades as steep as 1:14].'
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