Operating Era: 1922-1970. Pennsylvania RR. PRR designed the first quad hopper in 1909 for coke service, the class H21. It had clamshell hoppers and 50-ton trucks. Beginning in 1913 70-ton trucks were applied, creating the H21A class. Over 35,000 cars were built for PRR and the mines it serviced by 1917. Many lasted into the Penn-Central era. When the cars were given AB brakes in the early 1940's the interiors were revised to strengthen the crossridge stakes and remove the crossties and top chord stiffener.