Air Brake System Components and Operation
What this domain tests
The exam checks whether you can identify the major freight air brake components and explain how a brake pipe pressure change actually sets and releases the brakes, not just the names.
Core components
- Main reservoir and compressor: store and supply compressed air; the feed valve regulates brake pipe pressure in release.
- Automatic brake valve: controls brake pipe pressure; the equalizing reservoir is the reference pressure.
- Independent brake valve: controls locomotive brakes only.
- Brake pipe: the trainline signal; a reduction applies, an increase releases.
- Control valve (ABDX/ABDWO): senses brake pipe change and routes air from the auxiliary reservoir to the brake cylinder in service; the emergency reservoir supports emergency application.
- Cut-out cock, angle cock, dirt collector, retaining valve, slack adjuster: isolation, coupling, filtration, grade holding, and slack control.
How an application develops
A service reduction lowers brake pipe pressure. Each control valve senses the drop, moves to service position, and admits auxiliary reservoir air to its brake cylinder. The auxiliary reservoir equalizes with the brake cylinder, so a deeper reduction produces more cylinder pressure. Increasing brake pipe pressure releases: the control valve exhausts brake cylinder air and recharges the auxiliary reservoir.
Worked example
Brake pipe is charged to 90 psi. The engineer makes a 20 psi reduction, lowering brake pipe to 70 psi. Each car's control valve sees 20 psi less and feeds auxiliary reservoir air into the brake cylinder. The brakes set throughout the train; the rear sets later than the head end because the pressure change travels through the trainline. Release is made by increasing brake pipe pressure, not by moving the independent brake.
Traps that cost marks
Confusing the independent brake (locomotive only) with the automatic brake (whole train). Saying the brake pipe supplies brake cylinder air directly - it is the control signal; the auxiliary reservoir supplies the cylinder. Forgetting that the equalizing reservoir is the reference and that a cut-out cock isolates a car's control valve.