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.

In the twenties, the Great Northern needed powerful freight locomotives for the more than 900 mile-long stretch from Minot, North Dakota through Montana to Spokane, Washington. They needed lots of pulling power for the grades over Marias Pass, but enough speed for the long distance. The first four locomotives of class R-1 were delivered by Baldwin in 1925. They had the 2-8-8-2 wheel arrangement, four equal cylinders of 28 by 32 inches (711 by 813 mm) and drivers of 63 inches (1,600 mm).

At their boiler pressure of 210 psi, this resulted in a starting tractive effort of 142,165 pounds when using the usual formula. But since the Great Northern used a coefficient of .765 instead of .85, they only quoted a value of 127,950 pounds. The six-axle Vanderbuilt tender carried 16,800 gallons of water and 5,800 gallons of oil. Together, the locomotive and tender reached a total weight of nearly 450 tons.

Based on the first four R-1, 10 more were built in the GNR'S own Hillyard shops in 1927 and 1928. But these were not only heavier, but had their pressure raised by 15 psi and got a bigger firebox heating surface. While the Baldwin locomotives only had a combustion chamber, the new ones also got arch tubes which added 64 square feet. Also the older ones later got the arch tubes.

Since their operator still needed more power, they built 16 of the class R-2 in 1929 and 1930. The water capacity of the tender was increased to 22,000 gallons. At a total weight with tender of more than 500 tons, they were the biggest US locomotives built west of the Mississippi. The boiler pressure was again increased by 15 psi, the grate area was increased and the superheater was made much larger.

By 1946 they had been upgraded by increasing the firebox heating surface by 142 square feet through the addition of circulators and by an increased weight that led to a 24 tons higher adhesive weight. The combined surface of firebox, tubes, flues and superheater now reached a value of 11,349 square feet (1,054 m²). A special measure was the reduction of the diameter of only the front cylinders by two inches to match the steam consumption with the boiler.