According to the late Al Staufer, dean of Pennsy historians, the M-class Mountains were "the most favored and loved class of power on the Pennsylvania Railroad." They were the last and best of the Pennsy's home-grown steamers.Conceived as a dual-purpose passenger and fast freight engine, with the ability to replace L1s Mikados and double-headed K4s Pacifics, the M-Class was developed in typical conservative Pennsy fashion. The boiler design was based largely on the road's I1s 2-10-0's, and its KW trailing truck design was shared by over 1000 Pennsy Atlantics, Pacifics and Mikados.