From 1911 through 1917, 70-ton H21a "quad" hoppers were introduced for coal service. During 1922 and 1923 all early H21 hoppers were converted to H21a's by replacing the 50-ton trucks with 70-ton trucks equipping them for coal service and bringing the total to 36,000 H21a hopper cars. The original 1909 through 1930 quad hoppers were built with the "old style" double-door configuration. During the 1930s and 40s, all old-style double-door hoppers were were replaced with the "saw tooth" hopper, increasing capacity by 39 to 40 cubic feet -- making them easier to maintain.
 
Kits are based on original PRR drawings and include injection-molded plastic bodies, hopper doors, wine door latches with door bracing, coal load, interior bracing, end- or top-mounted brakewheel, X2f couplers, one-piece molded trucks and 33" chilled wheels with RP-25 flanges on brass axles.