The AC-9 was one of two Southern Pacific Railroad's articulated steam locomotive class that ran smokebox forward after 1920. Twelve AC-9 class locomotives were built by Lima in 1939 and were Southern Pacific's largest and heaviest steam engines, partly a consequence of low quality coal these engines were designed to burn. The AC-9s were partially streamlined (the only articulated steam engines to be so equipped), having "skyline casings" inspired by Lima's GS series of 4-8-4's, also made for Southern Pacific, and were equipped with coal tenders, unlike the cab forwards. The wheel arrangement was 2-8-8-4, which was nicknamed "Yellowstone". Between 1939 and 1952, all the twelve AC-9 engines were in service between Tucumcari, New Mexico, El Paso, Texas, and Tucson, Arizona, where they mainly