In order to prevent individual chassis groups from slipping on slow freight trains, the Great Northern ordered a Mallet with unequal axle distribution. The front group consisted of a leading axle and three driving axles, while the rear group consisted of four driving axles. After ten locomotives from 1909, another 25 followed, which had a significantly larger tube heating area. Under the new scheme, they were designated class M-1.

They were converted to simple expansion to save complexity from 1926 and were designated class M-2. The rear cylinders remained slightly larger in order to be able to convert enough power for the additional driving axle. The existing feedwater heater was also removed. The locomotives had a choppy running that made speeds over 25 mph (40 km/h) difficult to run. 22 were scrapped from 1929 and their parts used for new O-7 class Mikados, while the remaining 13 partially survived until 1954.

The Great Northern was one of the first railroads in the United States to order Mountain locomotives. The manufacturer Lima saw these as Mikados, which had a two-axle forward bogie for better running characteristics in passenger service. The Great Northern wanted to use the P-1 to eliminate the need for helpers on passenger trains in Montana and the Cascades.

They had a conical boiler with a Belpaire firebox. The first ten P-1s were coal-fired and the last five were oil-fired. The diameter of the drivers was only 62 inches or 1,575 mm, which was an advantage on the steep lines. The speed of passenger trains was now 20 mph (32 km/h) on the 1.8 percent in Montana and 15 mph (24 km/h) on the 2.2 percent in the Cascades, both without helpers.

When these locomotives were no longer sufficient to meet the increased requirements, in 1928 all P-1s were rebuilt into 2-10-2 freight locomotives and designated class Q-2. The drivers and the cylinder diameter were each increased by one inch. The boiler only remained the same on the outside and had many changes on the inside, as well as a higher pressure. They were all coal-fired, but nine were converted to oil in the 1940s. Their decommissioning began in the 1950s and was completed in April 1958.

The first Mallet with the 2-6-6-2 wheel arrangement was the L-1 class of the Great Northern. They were created in 1906 by adding a leading and a trailing axle to the 0-6-6-0 in order to improve running in curves. They were used as helpers in the Cascades. After the first five, 20 more followed in 1908 after the L-2, which had been delivered in the meantime, turned out to be too light. The class Z of the Northern Pacific was identical to these.

Although they had a Belpaire firebox, its heating surface was very small compared to the total heating surface. At least two L-1s received a superheater around 1915, and they also received significantly larger high-pressure cylinders. However, as early as 1925 and 1926, all L-1s were rebuilt to class O-6 Mikados, which remained in use until the mid-1950s.

Compared to the S-1, the locomotives of the S-2 class received larger driving wheels with a diameter of 80 inches, as they were only intended for express trains. They also burned oil and now had cylinders measuring 29 by 29 inches. They showed up more than 50,000 pounds less on the scales thanks to a smaller boiler. Since the adhesive weight was also reduced as a result, the locomotives slipped more quickly when starting and had to be operated with great care. Nevertheless, from 1949 they were no longer needed for express trains such as the “Empire Builder” and also had to pull freight trains.