In the 1920s, Southern Pacific rethought the standard approach to motive power; rather than changing engines at division points, they wanted to develop an engine for use on extended runs. Their famous 4-8-4 GS-series of locomotives resulted. 4-8-4 Northerns originated with the Northern Pacific railroad in 1926 and quickly became popular on other roads. The eight drivers allowed the engines to generate more power than engines with fewer driving wheels, and the four trailing wheels supported a larger firebox, increasing boiler capacity. This combination allowed 4-8-4s to best even larger 2-10-2-types by sustaining higher speeds over longer distances.

Southern Pacific's famous GS-4 was introduced in the 1930s specifically for the glamorous new "Daylight" passenger train that ran between San Francisco and Los Angeles. The engine had to be able to pull a 12-car, 626-ton train at high speeds, over long distances and grades. A 4-8-4 was the perfect solution to the problem.