Air Brakes and Train Handling Rules - Alaska Railroad

67 Car Equipment and Components67.8.2 Control Valve TypesMost freight cars have AB, ABD, or ABDW control valves.A. AB Control ValveIntroduced in 1933, the AB control valve is found on less than 10percent of the freight car fleet. The AB control valve introduced thequick service feature, which allows brake pipe pressure to ventlocally at each car during the initial service application. As a result,brakes apply evenly and lightly throughout the train.B. ABD Control ValveIntroduced in 1964, the ABD control valve includes quick serviceand is designed to improve reliability and reduce maintenance. TheABD control valve also includes accelerated service release, whichhelps recharge the brake pipe from the emergency reservoir on eachcar and rapidly transmits a release through the train.C. ABDW Control ValveIntroduced in the 1970s, the ABDW control valve includes all of thefeatures of the ABD valve plus the accelerated application feature.67.8.3 Control Valve FeaturesA. Quick ServiceThe quick service feature:• Allows 4 to 6 psi of brake pipe air to vent to the atmosphere ateach car• Allows quick service venting only when the control valve movesfrom RELEASE to SERVICE (Quick service does not allowlocal venting after the initial reduction is made or when thecontrol valve moves from LAP to SERVICE during furtherreductions.)• Includes a valve that limits brake cylinder pressure to 10 to12 psi67-8 ARRC Air Brakes and Train Handling Manual June 1, 1996