The Southern Pacific AC-9 is one of two classes of articulated locomotives that ran smokebox forward on the Southern Pacific Railroad. The class was numbered 3800 to 3811.
The AC-9 locomotive was one of two classes of articulated locomotives that the SP ran and operated smokebox forward. Twelve engines were built by Lima in 1939. This class of engine was one of the heaviest and largest classes operated by the SP. Their wheel arrangement of 2-8-8-4 meant they were "Yellowstones" but they were rarely called that. These were different form the SP other articulateds as these engines weren't cab forwards. Another difference was these engines were semi- streamlined which was odd for a freight hog.
The engines were assigned in the SP's South-Western Division and occasionally pulled the Golden State Limited.
In 1952, all twelve were converted from coal to oil burners and were moved to the Modoc line. There they worked on freight service until they were withdrawn in 1956. All locomotives of this class were scrapped.