Editing 4-8-4 (section)

===Demise===
The big-wheeled {{nowrap|4-8-4}} was at home on heavy passenger trains and quite capable of speeds over {{convert|100|mph|km/h|abbr=off}}, but freight was the primary revenue source of the railroads; in that service the Northern had limitations. The adhesive weight on a {{nowrap|4-8-4}} was limited to about 60% of the engine's weight, not including the dead weight of the tender. Henry Bowen, the Chief Mechanical Engineer of the [[Canadian Pacific Railway]] (CPR) from 1928 to 1949, tested the first two CPR K-1a Northerns introduced by his predecessor, then he designed a [[2-10-4|{{nowrap|2-10-4}} Selkirk]] type using the same boiler. The resulting T-1a [[Selkirk locomotive]] had the same number of axles as the Northern, but the driving wheels were reduced from {{convert|75|to(-)|63|in|m}} in diameter, while the additional pair of driving wheels increased the tractive effort by 27%. In a later variant, Bowen added a booster to the trailing truck, enabling the Selkirk to exert nearly 50% more tractive effort than the similar-sized K-1a Northern.

When it was demonstrated that a three-unit [[EMD F3]] diesel-electric [[consist]] that weighed slightly less than the total engine and tender mass of a CPR K-1a Northern could produce nearly three times its tractive effort, high-powered steam locomotives were retired as quickly as finance allowed.