Southern Pacific class AC-9

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Southern Pacific AC-9
Type and origin Power type Steam Builder Lima Locomotive Works Serial number 7765–7776 Build date 1939 Total produced 12
Specifications Configuration: ​  • Whyte 2-8-8-4  • UIC (1′D)D2′ h4 Gauge 4 ft 8+1⁄2 in (1,435 mm) Driver dia. 63+1⁄2 in (1,613 mm) Axle load 66,400 lb (30.1 tonnes) Adhesive weight 522,200 lb (236.9 tonnes) Loco weight 677,200 lb (307.2 tonnes) Total weight 998,000 lb (453 t) Fuel type Oil (originally coal) Fuel capacity 28 short tons (25 t; 25 long tons) Water cap. 22,100 US gallons (84,000 L; 18,400 imp gal) Firebox: ​  • Grate area 139.5 sq ft (12.96 m2) Boiler pressure 250 psi (1.72 MPa) Heating surface 6,918 sq ft (642.7 m2) Superheater: ​  • Heating area 2,831 sq ft (263.0 m2) Cylinders Four (simple articulated) Cylinder size 24 in × 32 in (610 mm × 813 mm) Valve gear Walschaerts
Performance figures Tractive effort 123,364 lbf (548.75 kN)
Career Operators Southern Pacific Company Class AC-9 Numbers 3800–3811 Nicknames Reverse cab forward Disposition All scrapped

The AC-9 was one of two Southern Pacific Railroad's articulated steam locomotive classes 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 1953, all the twelve AC-9 engines were in service between Tucumcari, New Mexico, El Paso, Texas, and Tucson, Arizona, where they mainly pulled freight trains and occasionally also passenger trains such as the Golden State Limited. Between May and August 1950, they were converted to burn oil instead of coal, and in spring 1953 eleven of them moved to Southern Pacific's Modoc line between Sparks, Nevada, and Alturas, California, where they worked in freight service from 1953 until retirement in early summer 1956.^{[citation needed]}

All examples were scrapped in 1956.