A Southern Pacific class AC-9 2-8-8-4 steam locomotive.

(Bob Fremming-Dallas, Wisconsin, Public domain, via Wikimedia Commons)


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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.

All examples were scrapped in 1956.


Southern Pacific AC-9 Overview

Type and origin
Power type: Steam
Builder: Lima Locomotive Works
Serial number: 7765–7776
Build date: 1939
Total produced: 12
​• Whyte 2-8-8-4
• UIC (1′D)D2′ h4
Gauge: 4 ft 8+1⁄2 in (1,435 mm)
Driver diameter: 63+1⁄2 in (1,613 mm)
Axle load: 66,400 lb (30.1 tonnes)
Adhesive weight: 522,200 lb (236.9 tonnes)
Locomotive weight: 677,200 lb (307.2 tonnes)
Total weight: 998,000 lb (453 t)
Fuel type Oil (originally coal)
Fuel capacity: 28 tons (25 t; 25 long tons)
Water capacity: 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)
Operators: Southern Pacific Company
Class: AC-9
Numbers: 3800–3811
Nicknames: Reverse cab forward
Disposition: All scrapped