The vacuum brake is a braking system employed on trains and introduced in the mid-1860s. A variant, the automatic vacuum brake system, became almost universal in British train equipment and in countries influenced by British practice. Vacuum brakes also enjoyed a brief period of adoption in the United States, primarily on narrow-gauge railroads. Their limitations caused them to be progressively superseded by compressed air systems starting in the United Kingdom from the 1970s onward. The vacuum brake system is now obsolete; it is not in large-scale usage anywhere in the world, other than in South Africa, largely supplanted by air brakes.

In the earliest days of railways, trains were slowed or stopped by the application of manually applied brakes on the locomotive and in brake vehicles through the train, and later by steam power brakes on locomotives. This was clearly unsatisfactory, given the slow and unreliable response times (each brake being separately applied by a member of the train crew in response to signals from the driver, which they might miss for any number of reasons, and necessarily in sequence rather than all at once where there were more brakes than crew members, making emergency braking extremely hit-and-miss) and extremely limited braking power that could be exerted (most vehicles in the train being wholly unbraked, and the power of all but the locomotive's own brakes relying on the strength of a particular crewmember's arm on a screw handle), but the existing technology did not offer an improvement. A chain braking system was developed, requiring a chain to be coupled throughout the train, but it was impossible to arrange equal braking effort along the entire train.

A major advance was the adoption of a vacuum braking system, in which flexible pipes were connected between all the vehicles of the train, and brakes on each vehicle could be controlled from the locomotive. The earliest scheme was a simple vacuum brake, in which vacuum was created by operation of a valve on the locomotive; the vacuum actuated brake pistons on each vehicle, and the degree of braking could be increased or decreased by the driver. Vacuum, rather than compressed air, was preferred because steam locomotives can be fitted with ejectors; venturi devices that create vacuum without moving parts.

The simple vacuum system had the major defect that in the event of one of the hoses connecting the vehicles becoming displaced (by the train accidentally dividing, or by careless coupling of the hoses, or otherwise) the vacuum brake on the entire train was useless.

In response to this obvious defect, the automatic vacuum brake was subsequently developed. It was designed to apply fully if the train became divided or if a hose became displaced. The automatic vacuum brake was slightly more expensive to manufacture and install than the simple system due to it requiring a higher number of machined parts, and incurred higher running costs since the ejector ran continuously (at a cost in steam and thus fuel and water) to maintain the vacuum when the train was running, rather than only being used when braking as in the simple system.

Opposition to the fitting of the automatic type of brake on the grounds of cost (particularly by the LNWR and its chairman Richard Moon - as one of the largest British companies, the refusal of the LNWR to adopt the automatic brake was a strong factor against its wider use by other companies, especially those interchanging traffic with the North Western) meant that it took a serious accident at Armagh in 1889 before legislation compelled the adoption of the automatic system. In this accident at Armagh, a portion of a train was detached from the locomotive on a steep gradient and ran away, killing 80 people. The train was fitted with the simple vacuum brake, which was useless on the disconnected portion of the train. It was clear that if the vehicles had been fitted with an automatic continuous brake, the accident would almost certainly not have happened, and the public concern at the scale of the accident prompted legislation mandating the use of a continuous automatic brake on all passenger trains.

In continental Europe, the vacuum brake was sometimes called the Hardy brake,[1] after John George Hardy of the Vacuum Brake Co, 7 Hohenstaufengasse, Vienna.

In its simplest form, 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 at atmospheric 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 on the treads of the wheels.