Early railroad cars had hand-operated brakes. A lever (and later a brake wheel) near the top of each car applied the brakes only on that individual car. The engineer directed brakemen to apply or release brakes by whistle signals; crewmen scrambled from rooftop to rooftop, leaping the gap between cars. An untold number of railroaders died during this terribly hazardous process.
George Westinghouse demonstrated the first practical air brake system in 1868. A locomotive supplied compressed air to pistons on each car, forcing brake shoes against the wheels. Westinghouse's later Automatic Air Brake was far more "fail-safe," as the loss of air pressure due to a system failure, uncoupling, or derailment applied the brakes on all cars automatically. The system was first used on a freight train in 1884, and (with various improvements) has been in general use since that time. Cars still have hand-operated brakes, but only for use when cars are not coupled to a train's air line.
Westinghouse Automatic Air Brakes
George Westinghouse's Automatic Air Brake system, developed in 1872, was railroading's most significant safety advance.
A compressor on the locomotive supplies compressed air to a brake pipe running beneath each car and through flexible hoses between cars. The brake pipe supplies air to a reservoir on each car. When the engineer uses his brake valve to reduce pressure in the brake pipe, a valve on each car senses the pressure change and uses the air in the car's reservoir to apply the brakes proportionately. A 5-10 psi reduction in the brake pipe pressure applies the brakes lightly, while a 20 psi or larger reduction applies the brakes heavily.
If a brake pipe or hose ruptures due to a derailment or uncoupling, the sudden loss of pressure in the brake line causes a near-instantaneous emergency (maximum) brake application on each car. The engineer can also make an emergency brake application by moving his brake valve to the emergency, or "big hole," position.
Modern Train Braking Systems
Trains use two separate air brake systems. Both use compressed air from the locomotive, and are controlled from the cab.
The engineer's automatic air brake valve controls the brakes on both the locomotive(s) and cars. The automatic brake valve reduces the pressure in the brake pipe connected to each car's air reservoir, applying the locomotive's and cars' brakes in proportion to that pressure reduction. On heavy downhill grades, a "retainer valve" on each car can be manually set to keep the brakes applied.
The independent air brakes are controlled by a separate valve in the cab. The independent valve only applies the brakes on the locomotive(s), not the cars.
A skilled engineer uses a combination of automatic and independent brakes to keep his train under control and to prevent damage as the train slows or goes up and down grades.