vacuum brake
A brake used on trains, working on the principle of keeping up a vacuum in a continuous pipe or pipes extending under the train, and in brake-cylinders connected to them under each vehicle, the air being sucked out by ejectors or pumps on the locomotive.
vacuum brake: air-pressure braking via negative pressure
A vacuum brake is a pneumatic braking system that operates by maintaining a partial vacuum in a continuous pipe running the length of a train. When air is removed from brake cylinders mounted under each carriage or wagon, atmospheric pressure above the piston pushes it downward, engaging brake shoes or blocks against wheels or drums. This contrasts with compressed-air brakes, which use positive pressure to apply force. Vacuum brakes were the standard on British railways and remain in use on heritage lines, though they have been largely superseded by more responsive air brake systems.
The vacuum is generated and maintained by ejectors or pumps fitted to the locomotive. Early systems used steam ejectors that exploited the engine's boiler pressure to create suction; later diesel and electric locomotives used motor-driven vacuum pumps. The pipe network must be continuous and sealed, connected between vehicles via flexible hoses with quick-release couplings. Each vehicle carries a brake cylinder, typically 20 to 24 inches in diameter, with a piston that moves to actuate a brake lever or rod when vacuum is lost or reduced.
To apply brakes, the driver gradually vents air into the pipe using the brake valve in the locomotive cab, reducing vacuum throughout the train. This is a gradual, even process: all cylinders lose vacuum at roughly the same rate, so all brakes apply together. This smooth application made vacuum brakes valued for passenger trains, where sudden stops cause discomfort. However, the system lacks a fail-safe advantage: if the pipe fractures, vacuum is lost and all brakes apply immediately, which can be dangerous on moving trains in poor visibility.
Limitations and decline
Vacuum brakes respond more slowly than compressed-air systems because the pressure differential driving the piston is limited to approximately 14.5 pounds per square inch (atmospheric pressure minus vacuum, typically around 21 inches of mercury). Modern trains require faster, stronger braking power. Additionally, vacuum brakes cannot be used on multiple-unit trains without complex modifications, making them impractical for modern railway networks. The system also demands tight sealing throughout the pipe network, and small leaks compound quickly across long trains.
The term reflects the system's fundamental principle: the brake is powered entirely by the absence of air rather than its presence. Vacuum brakes appear in period equipment, heritage railways, and older industrial rail systems. They remain educationally important as the ancestor of modern pneumatic brake designs and as a working example of how negative pressure can be harnessed for mechanical work.