The 4-10-2 was American Locomotive Company's three-cylinder development of the 2-10-2, the extra weight of the third (middle) cylinder requiring a four-wheel pilot truck. Only two railroads, the Southern Pacific and the Union Pacific, bought the design after ALCo demonstrated it in 1925. The SP owned the majority of them, 49 engines delivered 1925-27 in three groups, and designated them the "Southern Pacific" type. (The UP's ten were styled the "Overland" type.) Although originally intended for dual service over Donner Pass, the 4-10-2s were reassigned to freight service on other SP divisions when it was found that, with their long rigid wheelbase, they had trouble negotiating the curves through the Sierras. No. 5012 of SP's class SP-1 appears here at Los Angeles on August 27, 1952 in a photo by an unknown lensman contributed by Carl Weber. For specifications of the SP type consult the commentaries for Nos. 5025 and 5045 following.