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A vacuum brake served as the universal standard braking system on railway trains until it was replaced by the more efficient and safe compressed air systems used today. The vacuum brake system functions by inducing a vacuum in a series of hoses which run throughout the train. These hoses are connected to piston units on each carriage which, when actuated by the vacuum, close brake calipers on the carriage wheels. On older steam trains, the vacuum was supplied by ejectors on the locomotive while newer systems utilized electrically driven vacuum pumps. Although compressed air systems are now the norm on most new trains, there are still rail operators using rolling stock with combination compressed air/vacuum brakes.
Although generally effective, simple vacuum brake systems have one serious, sometimes fatal flaw, i.e., any break in the hoses render the entire system inoperative. This damning "Achilles' heel" led to several serious accidents involving runaway trains and the eventual installation of the safer automatic vacuum brake system. Automatic vacuum systems locked up all of a train's brakes if there was any interruption of the vacuum supply. Although safer, the systems were still slow to initialize especially in the case of long trains. These performance issues slowly led to the widespread installation of compressed air braking systems which are quicker to actuate and release and generally safer.
The vacuum brake system has, at its heart, a central vacuum line or pipe which runs the length of the train. The line is typically a steel pipe on the carriages joined with flexible hoses between the individual carriages. The last carriage has a blank fitting inserted at the end of its vacuum line to close the circuit. On steam locomotives, an ejector supplies the vacuum for the brakes and is regulated by a series of levers in the drivers cab. These ejectors are simple steam operated venturi devices which draw sufficient negative pressure to actuate the vacuum brake system.
When braking force is required, the train driver activates the ejector which draws air out of the central line, thereby creating a vacuum in the system. A braking piston assembly is mounted on the underside of each carriage and connected to the central vacuum line. When moved by the negative pressure in the system, this piston activates a series of linkages which pulls the break shoes up against the carriage wheels. When the brakes are released, the vacuum reduces and air flows back into the piston cylinder moving the it back and reopening the brake caliper to release the wheels.