The vacuum brake system was the standard on trains until it was replaced by compressed air systems. The system works by inducing a vacuum in hoses that run throughout the train, but any break in the hoses renders the system inoperable. The system is still used by some rail operators with combined compressed air/vacuum brakes. The system has a central vacuum line that runs the length of the train and is typically steel pipe on the cars with flexible hoses between them. When braking force is required, the train driver activates the ejector which draws air from the center line, thus creating a vacuum in the system.
A vacuum brake served as the universal standard braking system on railway trains until it was superseded by the more efficient and safer compressed air systems in use today. The vacuum brake system works by inducing a vacuum in a series of hoses that run throughout the train. These hoses are connected to piston units on each cart which, when activated by vacuum, close the brake calipers on the cart’s wheels. On older steam trains, vacuum was supplied by ejectors on the locomotive, while newer systems used 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 combined compressed air/vacuum brakes.
Although generally effective, simple vacuum brake systems have a serious, sometimes fatal flaw, that is, any break in the hoses renders the entire system inoperable. This cursed “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 all the brakes on a train if there was any interruption in the vacuum supply. Although safer, the systems were still slow to start, especially for long trains. These performance issues slowly led to the widespread installation of compressed air braking systems that are quicker to actuate and release, and generally safer.
The vacuum brake system has, at its heart, a central vacuum line or pipe that runs the entire length of the train. The line is typically steel pipe on the cars attached with flexible hoses between the individual cars. 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 driver’s cab. These ejectors are simple steam-operated venturi devices that generate enough negative pressure to actuate the vacuum brake system.
When braking force is required, the train driver activates the ejector which draws air from the center line, thus creating a vacuum in the system. A brake piston assembly is mounted on the bottom of each carriage and is connected to the central vacuum line. When moved by the negative pressure in the system, this piston activates a series of links that pushes the shoes against the carriage wheels. When the brakes are released, the vacuum is reduced and air flows back into the piston cylinder moving it back and reopening the brake caliper to free the wheels.
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