At the beginning of the 20th century when the Union Pacific, Southern Pacific, Kansas City Southern and others were under the same ownership, several locomotive types were standardized in the Harriman Common Standard. One of the HCS Atlantics was the class A-3 of the Southern Pacific. 47 had been built between 1904 and 1908 by ALCO and Baldwin, of which most were oil-fired. They are said to have hauled the Daylight Limited over the 223 miles (359 km) from Los Angeles to San Luis Obispo without a refueling.

Beginning in the twenties, all were superheated. In this process, most got a feedwater heater and a trailing truck booster. In the example of the four locomotives rebuilt by the Texas & New Orleans, they even got smaller drivers with a diameter of 77 instead of 81 inches. Another four were rebuilt to the even more modern class A-6 in 1927.