For mixed service in front of 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 thermosiphons, fire bricks and water pipes. What was special about the rear bogie was that the wheels on the two axles had very different diameters of 34 and 48 inches.

The first delivery from 1927 included 20 units each from the Canadian Locomotive Company and the Montreal Locomotive Works, which were designated as class U-2a and U-2b. The frame of both series was partly made of vanadium steel and nickel steel. Some had a booster on a rear bogie axle that provided an additional starting tractive effort of 10,900 pounds. ALCO built ten more, almost identical locomotives for the Grand Trunk Western.

From 1929 a new version was produced in which the relationship between the flues and tubes in the boiler differed from the first series. The frames were now only made of nickel steel. Instead of the spoked wheels, the lighter boxpok wheels were now used. The first 20 examples of this design came from Montreal and were designated U-2c. From there came five more as U-2d in 1936 and 15 as U-2e in 1940. Also in 1940, ten U-2fs arrived from the Canadian Locomotive Company.

U-2d No. 6161. The Boxpok wheels can be seen in comparison with the U-2a

collection Taylor Rush

The last version had a smaller tube heating surface than the second version and therefore a larger superheater. The first 35 were completed in 1943 by the Montreal Locomotive Works as the U-2g. Later that year and in 1944, another 30 were made at the same plant, known as the U-2h.

One example still in existence today is U-2g No. 6213. It was used until 1959 and is now owned by the City of Toronto. It has been displayed at Exhibition Place and has been cared for by the Toronto Locomotive Preservation Society over the years. In 2009 she was transferred to a roundhouse, where she was cosmetically prepared in 2019. In its current condition, it is quite conceivable that it could be restored to operational condition in the future.

Another U-2g that still exists is number 6200 in Ottawa. This was recently rebuilt so that it now can be towed. Number 6218 sits in Fort Erie, Ontario but hasn't been restored yet.