All A-3 Atlantics built from 1904 onwards were fitted with a superheater from 1920 and most of them also received a feedwater heater and a booster on the trailing axle. Four underwent an even bigger rebuild in the Southern Pacific workshops, resulting in the class A-6. Two each were completed in August 1927 and October 1928.

As opposed to other rebuilds with a driver diameter of 77 inches, the A-6 retained the original 81 inches. In addition, the diameter of the cylinders was increased by two inches and the boiler pressure was increased by ten psi. The tube heating surface has been increased by about five percent and the frame has been strengthened. In total, this resulted in a service mass increased by around 20 tons. Their duties included the haulage of the Sacramento Daylight.