Distinguished by its being the only North American railroad besides the Pennsylvania to standardize on the use of the Belpaire firebox, the Great Northern had some extremely interesting designs. The GN received its first Mikados in 1911, class O-1, a class of 145. Originally built with 180 psi boiler. Later 210 psi, this was a "heavy" class, 63 inch drivers and a starting tractive effort of almost 61,000 lbs, with booster fitted in many. This was followed by a single merger acquired class O2 locomotive, then with 9 USRA 2-8-2B, class O3, then a more powerful version of O1, class O4, 45 locomotives in 1920, with the starting tractive effort over 64,000 lbs.

The Great Northern was an early user of articulateds, starting with Mallets. Then it turned to simples, winding up with class R2 of 1929, a four cylinder 63 inch driver simple 2-8-8-2, company built which, in its final iteration with a new boiler in 1947 was one the truly outstanding designs ever built in North America. But, one of the early Mallets was 2-6-6-2 class L-2, built in 1908. Starting in 1925, these locomotives were dismantled, "parts" being used in the construction of 45 small 2-8-2s of class O5. These had 63 inch drivers, 25 x 30 cylinders, 200 psi boilers and a starting tractive effort of 50,600 lbs. Weights were 220,000 lbs on drivers, 283,000 total.

This rebuilding was so successful that the GN then rebuilt an earlier (1907) but larger class of Mallet 2-6-6-2, L2, into 22 class O6 locomotives. This was done in 1925/26, and resulted in a much larger Mikado: 28 x 32 cylinders, 63 inch drivers, 200psi, weights 244/320,000 lbs, starting tractive effort 66,000 lbs.

In 1909, the Great Northern had installed 35 class M1, Mallets of an unusual wheel arrangement, 2-6-8-0, In 1924/5, the road rebuilt the class as simple engines, class M2. Not satisfied, the GN in 1925/6 proceeded to rebuild 22 of the M2 as a new, and very large 2-8-2, class O7. These locomotives took the GN into the field of truly giant 2-8-2s: 31 x 32 cylinders, 69 inch drivers, 210 psi, 268/348,000 lbs weight, 79,950 lbs tractive effort. Seemingly, the GN was not satisfied with the O7. New boilers were ordered from Baldwin and Alco, and the result was class O8 of 1945/6: 28 x 32 cylinders, 250 psi boilers, 69 inch drivers, weights 325/425,000 lbs, starting tractive effort 75,900 lbs. (In 1932, 3 earlier O8s had been built from scratch. They had 29 x 32 cylinders, 69 inch drivers, 250 psi boilers, weights of 280/367,000 lbs. Starting tractive effort was 78,000 lbs. In 1946, these three locomotives were rebuilt to conform to the 22 "new" O8s, thus making a class of 25. Also, over time, some if not all of the class were converted to oil burners.)

The Great Northern had equipped the final version of the O8 with roller bearings and cast steel engine beds, thus bringing them more up to date that the two classes of 4-8-4 on the road, dating from 1929 and 1930. In fast freight service, service, over a 275 mile district with a opposing grade of 0.4% (in two locations, higher grades were handled with the assistance of helpers), pooled with 4-8-4s, the O8 more than held their own. On the other hand, the comment from one knowledgeable person, Patrick C. Dorin, in his book, "Lines East" was "Many operating people were not overly pleased with the performance of the O8, despite its power." In any case, it was a most interesting design -- the largest Mikado ever built.

There is one surviving Great Northern 2-8-2 "Mikado" type locomotive. It is a Class O-1, number 3059, on display in a city park in Wiliston, ND.