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Specifications
Specifications
Configuration:;
• Whyte	4-8-8-2 simple articulated
Gauge	4 ft 8+1⁄2 in (1,435 mm) standard gauge
Driver dia.	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)

Type and origin
Type and origin
Power type	Steam
Builder	Baldwin Locomotive Works
Serial number	70082–70101
Build date	October 1943 – March 1944
Total produced	20

Southern Pacific class AC-12
Southern Pacific class AC-12
	Southern Pacific 4294 on display at the California State Railroad Museum
Power type
Type and origin
Power type	Steam
Builder	Baldwin Locomotive Works
Serial number	70082–70101
Build date	October 1943 – March 1944
Total produced	20
Configuration:;
Specifications
Configuration:;
• Whyte	4-8-8-2 simple articulated
Gauge	4 ft 8+1⁄2 in (1,435 mm) standard gauge
Driver dia.	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)
Tractive effort
Performance figures
Tractive effort	124,300 lbf (553 kN)
Operators
Career
Operators	Southern Pacific Railroad
Class	AC-12
Numbers	4275–4294
First run	October 27, 1943
Retired	1955–1958
Disposition	One preserved (#4294); remainder scrapped

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. Each locomotive was 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.