While most of Southern Pacific's later articulated locomotives were Cab Forwards, the twelve AC-9s were of conventional design. Since there were no tunnels to pass through on the line from Tucumcari, New Mexico to El Paso, Texas, they were able to burn coal in the usual way. Since the coal available was not of high quality, the grate measured 139.5 square feet or just under 13 m².
These Yellowstones were the heaviest locomotives that the Southern Pacific operated. They had a skyline casing on the top of the boiler and were designed for speeds of 75 mph or 121 km/h. After their traditional line was converted to diesel, they were converted to run on oil and moved to other lines. They were retired between 1953 and 1956. 