The classes U-1-a and U-1-b of the Canadian National consisted of a total of 37 Mountains with a driver diameter of 73 inches. The a and b subclasses were essentially identical, but U-1-a designated the 16 locomotives built in 1923 and U-1-b designated the 21 built in 1924. All U-1-a and some U-1-b were built with a Robinson superheater, while the rest got one of the Schmidt design. Later, more were rebuilt to the Schmidt variant. They were retired between 1951 and 1962. Today, only U-1-a is still existing and on display at Jasper, Alberta. 