The Canadian National Railways was formed in 1923 to rescue several financially troubled rail lines. At the time of the merger new motive power was badly needed and the CNR's first order for new locomotives was given to the Canadian Locomotive Company to build sixteen "Mountain" type locomotives.

This batch of sixteen 4-8-2s were delivered in 1923 and was designated as Class U-1-a and assigned road numbers 6000 through 6015. These locomotives had 26 x 30 cylinders, 73" drivers, a 210 psi boiler pressure and exerted 49,588 lbs of tractive effort. They weighed 354,110 lbs and were used on express passenger trains between Montreal and Toronto.

Twenty-one more "Mountains" were delivered from the Canadian Locomotive Company during 1924 and 1925. These locomotives were designated as Class U-1-b and given road numbers 6016 through 6036. They were similar to the Class U-1-a "Mountains" and were added to the passenger motive power roster.

Five more 4-8-2s (Class U-1-d, road numbers 6042 through 6046) from the Canadian Locomotive Company and twelve more 4-8-2s (Class U-1-e, road numbers 6047 through 6058) from the Montreal Locomotive Works were delivered in 1929 and 1930. These last two groups had minor improvements but were essentially the same as the sixteen delivered in 1923.

A final batch of twenty "Mountains" were delivered by the Montreal Locomotive Works in 1944. This group was designated as Class U-1-f and assigned road numbers 6060 through 6079. These very heavy (416,500 lbs) locomotives were semi-streamlined and intended to be used primarily for fast passenger service. Some were also used in a dual freight and passenger capacity. They were capable of reaching speeds up to 100 miles per hour.

The U-1-f class locomotives had a distinguising conical nose which housed the headlight and the number lamps. The stack was flaired after the British style. They were normally painted in CNR's passenger colors of black with green board skirts, cab and tender. Some of them were eventually converted to burn oil.

There are five surviving CNR "Mountains":

• 6015 at station yard in Jasper, AB
• 6043 at Assiniboine Park in Winnipeg, MB
• 6060 at the Rocky Mountain Railway Society in Calgary, AB
• 6069 at Bayview Park in Sarnia, ON
• 6077 at Prescott Park in Capreol, ON