In order to be able to accommodate a longer boiler on the Consolidation, the Norfolk & Western chose the 4-8-0 “Twelve-wheeler” wheel arrangement instead of the Mikado. The aim was to achieve a higher adhesive weight because the weight of the firebox was placed on the rear driving axles. Even the firebox was extended to the rear of the cab so that the engineer and fireman were separated from each other. They were nicknamed “Mollies” by the crews.

After 125 class M locomotives, 100 more class M1 locomotives were built. They only differed in the valve gear and the valves. Due to design errors, the M1 was subject to high levels of wear and tear, so it was phased out relatively early. The M was replaced on main lines by the 2-8-8-2 class Y and was used on secondary lines and in switching service until the late 1950s. Only ten had received a superheater. The number 475 is still in use on the Strasburg Railroad today and the 433 also survives.