In order to significantly increase speeds of heavy and fast freight trains, the C&O had the H-8 class developed by Lima. The inspiration came from the class A of the Norfolk & Western with the wheel arrangement 2-6-6-4. A three-axle trailing bogie ensured that an even larger firebox could be installed than on the existing locomotives. It was given relatively large drivers and steam heating so that it could pull passenger trains at 70 mph (113 km/h) if necessary.

The boiler was even larger than on the Big Boy and the locomotives were otherwise designed according to the latest standards. With an output of around 7,500 hp on the drawbar, the locomotives were among the most powerful steam locomotives ever. The installation of a booster in the bogie was rejected because even higher tractive forces were not required. There is some controversy regarding the actual weight of the H-8, as changes during development made it significantly heavier than originally planned. The first driving axle was loaded with 86,700 pounds or 39.3 t, which was the highest axle load of any steam locomotive.

The C&O named the wheel arrangement “Allegheny” after the mountain range that had to be overcome. Here two H-8 hauled up to 11,500 short tons. There they replaced the 2-8-8-2 class H-7 and enabled approximately twice the speed for heavy coal trains. On flatter routes they replaced the 2-10-4 T-1 class and could pull up to 13,500 short tons alone.

The first 45 H-8s were built during World War II, during which time they had to pull both urgent cargo and troop transports. The Virginian had eight AG class locomotives built in 1945, which were based on the H-8. In 1948 the C&O received another 15 H-8s, which were again similar to the AG. As early as 1952, the first H-8s were replaced by diesel locomotives. By 1956 they had all disappeared and today 1601 and 1604 remain preserved in static condition.