Southern Pacific 4294 on display at the California State Railroad Museum.

(Joe Ross, CC BY-SA 2.0 <>, via Wikimedia Commons)


BLW logo
SP logo


Southern Pacific Railroad's AC-12 class of simple articulated 4-8-8-2 cab forward steam locomotives was the last class of steam locomotives ordered by Southern Pacific. They were built by Baldwin Locomotive Works during World War II, with the first, number 4275, entering service on October 27, 1943, and the last, 4294, on March 19, 1944. The locomotives were effectively a conventional 2-8-8-4 locomotive running in reverse; the tender being coupled at the smokebox end of the locomotive. This was made possible by the use of oil-firing. The distinct features of these locomotives include: a streamlined front with white band, an air horn on the front, a streamlined pilot, a SP 12 wheel box tender, and air compressors mounted on the smokebox. Southern Pacific used these locomotives all over its system, but they were extremely famous for working on Donner Pass & Cascade Summit.

SP used the AC-12s for a little over a decade with the first retirements occurring on April 5, 1955, and the last on September 24, 1958. Only one AC-12, SP 4294, has survived into preservation and is now on display at the California State Railroad Museum.


No. 4294 was the last in a group of 20 locomotives that comprised Southern Pacific's AC-12 class of 4-8-8-2 cab forward locomotives.

(Orin Zebest, CC BY 2.0 <>, via Wikimedia Commons)


Southern Pacific class AC-12 Overview

Type and origin
Power type Steam
Builder Baldwin Locomotive Works
Serial number 70082–70101
Build date October 1943 – March 1944
Total produced 20
Configuration: Whyte 4-8-8-2 simple articulated
Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge
Driver diameter: 63 in (1,600 mm)
Adhesive weight 531,700 lb (241,200 kg; 241.2 t)
Loco weight 657,900 lb (298,400 kg; 298.4 t)
Total weight 977,900 lb (443,600 kg; 443.6 t)
Boiler pressure 250 psi (1.72 MPa)
Feedwater heater 6SA Worthington
Cylinders 4 (simple articulated)
Cylinder size 24 in × 32 in (610 mm × 813 mm) (bore × stroke)
Performance figures
Tractive effort 124,300 lbf (553 kN)
Operators Southern Pacific Railroad
Class AC-12
Numbers 4275–4294
First run October 27, 1943
Retired 1955–1958
Disposition One preserved (No. 4294); remainder scrapped


Southern Pacific: SP No 4294 walks a reefer block eastbound up Donner Pass at Blue Canon, CA. The 4294 was the last cab forward

built for the railroad and was delivered in 1944. It was retired in 1955 and donated to the city of Sacramento. Painting by Howard Fogg.

(ausdew, Public domain,