The class 3400 were the last Pacifics that the Santa Fe purchased. After many experiments had been attempted at the beginning of the century, these were now relatively simple machines with a simple two-cylinder engine. While the number of tubes in the boiler was based on the USRA Heavy Pacific, the firebox was only about the size of the Light Pacific.
Between 1936 and 1947, 44 of the 50 locomotives underwent a major rebuild. In addition to an oil-fired boiler and combustion chamber, they received a new, integrally cast frame. There were also now Boxpok drivers with a diameter of 79 instead of 74 inches, which ran in roller bearings.
Since the rebuilt locomotives were significantly more powerful, their lives were extended. As they were increasingly less needed for express service, they were more and more often used with freight trains. The retirements finally took place between 1950 and 1955. Five have been preserved, of which only 3415 had been operational since 2009 and is currently waiting for its scheduled 15-year overhaul.