For mixed service with freight, passenger and express trains, the Canadian National procured locomotives with the wheel arrangement 4-8-4 for the first time in 1927, known in English-speaking countries as “Northern”. They followed the “superpower” philosophy popular in North America at the time and boasted a grate area of more than 80 square feet thanks to the rear bogie. The heating surface of the firebox was maximized with thermic syphons, fire bricks and water tubes. What was special about the rear truck was that the wheels on the two axles had very different diameters of 34 and 48 inches.