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Handwriting on the back of this small, 4½x2¾-inch photograph (including borders) reads: "Southern Pacific - Nehwall, Calif. - June 37 / Swett."
But that's not possible. The depot (or the train) is on the wrong side of the tracks, relative to the town in the background, which is not Newhall.
"Swett" is probably Ira L. Swett (1913-1975), a railfan and rail photographer from Los Angeles who, by profession, was a publicist and administrator for the Salvation Army in Los Angeles.
SP Engine No. 3221 was built by Baldwin Locomotive Works in June 1913 (Robertson 1998:273). It was a Class Mk-4 "Mikado" type with a 2-8-2 wheel configuration — four pairs of coupled driving wheels with a two-wheeled leading and two-wheeled trailing truck. According to "History of the Baldwin Locomotive Works, 1831-1923," republished within Westing 1966, the Mikado "has come into extensive use on American railroads" since 1909, and "because of its increased steaming capacity, has largely replaced the Consolidation type for mainline service" (Westing:71).
A variety of the Mikado that burned lignite fuel was designed by Baldwin's J.F. Graham, Superintendent of Motive Power. It was built in 1910 for the Oregon Railroad and Navigation Company. This variety "proved highly successful, and the Mikado type locomotives subsequently built for the Union and the Southern Pacific [Railroads] and their associated lines were directly based upon it" (ibid.:71-72).
According to railfan Don Ross (donsdepot.donrossgroup.net), No. 3221 was retooled in 1917 and 1927. Some larger cylinders were installed in September 1937, and in October it was leased to a railroad in Mexico — which must have been shortly after this photograph was made. No. 3221 was returned in 1941, retired in 1953, and scrapped February 5, 1954.
LW3655: 9600 dpi jpeg from original photograph purchased 2019 by Leon Worden.