Although they had five driving axles, the 2-10-4 class T1 locomotives of the Canadian Pacific were intended for both passenger and freight service. They were designed to haul all kinds of trains over the Selkirk Mountains in British Columbia, which also gave the class its name. Another task was to work as helper for other trains on these lines. They were the largest and most powerful locomotives in the British Empire.

Despite their size, they were able to operate on lines with curve radii of 480 feet (146 m) and grades of 2.2 percent. Additionally to their starting tractive effort of 77,200 pounds, their trailing truck booster added another 12,000 pounds. They were oil-fired and had tenders with a capacity of 12,000 imperial gallons of water and 4,100 imperial gallons of oil. The first batch of 20 locomotives delivered by the Montreal Locomotive Works in 1929 was called class T1a and had no streamlining at all. The single experimental T4a high pressure locomotive was also based on the T1a.

In 1938, ten semi-streamlined T1b followed. These had a higher boiler pressure of 285 instead of 275 psi (19.7 instead of 19 bars). In turn the cylinder diameter was reduced by half an inch to get roughly the same tractive effort. Due to advancements in manufacturing, they were still lighter. In 1949, a final batch of six T1c was delivered which were nearly identical to the T1b, but had detail improvements like a second air pump. These were the last standard gauge steam locomotives built in Canada for a Canadian railway and two of this subclass are still existing.