For heavy ore trains with up to 115 cars, the Duluth, Missabe & Iron Range needed more powerful locomotives, where the most difficult task was to pull the empty trains up the 2.2 percent steep ramp from the port in Duluth to the marshalling yard in Proctor. The Western Pacific class M-137 was taken as the basis. With a two-axle trailing bogie, the wheel arrangement was 2-8-8-4 "Yellowstone", which allowed for a larger firebox and a larger all-weather cab for the cold Minnesota winters.

In 1941 eight members of the M-3 class were built and in 1943 ten more M-4s. Since the DM&IR had less traffic volume in winter, some of the locomotives were leased out. They were particularly valued by the D&RGW, where they had to pull heavy freight trains over a 10,239 foot high pass over the Continental Divide. With the exception of one locomotive that had an accident, all remained in use until they were replaced by diesel locomotives between 1958 and 1963. Today three remain, all of which are no longer operational.

In 1910, the DM&N initially received eight heavy Mallets of the 2-8-8-2 wheel arrangement, which they designated as class M and which largely corresponded to the class AB No. 600 of the Virginian. The intended area of operation was the ten miles from the docks in Duluth to Proctor at a gradient of 2.2 percent. Here the locomotive alone was to be able to push 55 empty ore wagons uphill at 12 mph. 

They were still operated with saturated steam, but had a second feedwater heater between the high and low pressure cylinders in addition to the regular one. By default, no stoker was installed, instead two firemen were used. While the 55 cars were a great challenge for the two firemen on the incline, locomotives with a retrofitted stoker could transport up to 85 empty ore cars. In the years 1916 and 1917, two more locomotives each followed, which were designated as class M-1 and M-2. These had a superheater from the factory and were five feet shorter overall.

Of the eight class M locomotives, seven were fitted with a superheater in 1925 and all those that did not yet have a stoker were retrofitted with one. The last locomotive, number 207, was built in 1930 in a total rebuild. In the completely new boiler, the superheater had a significantly larger share, resulting in a heating surface that was almost 60 percent larger overall. The cylinders were now of the same size without compound working. In the same year, the four M-1 and M-2 received a similar boiler with the larger superheater and four simple cylinders, which made them the M-2-S class.