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 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.

Only one AC-12, SP 4294, has survived into preservation and is now on display at the California State Railroad Museum.