This older braking technology relies on an ejector or pump on the locomotive to evacuate air from a continuous train pipe. When the engineer admits air into the pipe, atmospheric pressure pushes against a piston in each car's brake cylinder, applying the brakes. It was largely replaced in North America by the Westinghouse air brake system, which uses compressed air instead of a vacuum.
The museum still runs that old narrow gauge equipment with a working vacuum brake, so you have to handle it completely differently than a standard air setup.
Related terms
Brake pipe
The air line running the full length of a train that charges and controls every car's brakes.
Air line
A railroad route built as straight and level as possible between two points to minimize travel time.
Triple valve
The core control valve in a railway air brake system that charges, applies, and releases the brakes.