One of the USRA standard types intended for lighter, fast freight trains was the Heavy Mountain. Compared to the Light Mountain, it had the same wheel diameter and boiler pressure, but a cylinder diameter increased by one inch. Compared to the Mikados, which were also intended for lighter freight trains, they had a driver diameter that was seven inches larger and were therefore designed for higher speeds.
Only 15 of the Heavy Mountain were built, five of which were built as the class J-2 of the Chesapeake & Ohio and ten as the class K2 of the Norfolk & Western. A total of 37 copies were built in 1923 and 1926, differing in details from the USRA standard design. The Norfolk & Western's twelve K-2a already had higher boiler pressure and larger drivers.