Quick answer
Freight-train air brakes are designed so a reduction in brake-pipe pressure commands the cars to apply their brakes. Each car carries braking equipment and an air reservoir.
The brake pipe
A continuous air line runs through the train. The locomotive's compressor supplies air and the brake system uses changes in brake-pipe pressure to control applications and releases.
Why a broken train applies the brakes
A rapid, major loss of brake-pipe pressure—such as a train separation—can trigger an emergency brake application. This fail-safe principle is one of the defining features of automatic railway air brakes.
Long trains take skill to handle
Brake-pressure changes propagate through a long train over time. Engineers must account for train length, grade, slack and braking forces, and may also use dynamic braking on locomotives.
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