Question: What year did the Southern Pacific class AC-9 begin running between Alturas, CA and Nevada's fifth most populous city?

Context paragraphs:
Butte County Railroad: The Butte County Railroad was a 31.5 mi class II railroad that ran from a connection with the Southern Pacific Railroad at Chico, California to the Diamond Match Company lumber mill at Stirling City.  The railroad operated from 1903-1915 and then became the Southern Pacific's Stirling City Branch.  From 1915 until abandonment in the 1970s the line was operated as the Southern Pacific's Stirling City Branch.  The Chico and Northern Railroad was a non-operating subsidiary holding company of the Southern Pacific Railroad that was created to acquire a 32.31 mile line from Chico - Stirling City from the Butte County Railroad.  Upon acquiring the line, Chico & Northern immediately leased the line back to the Butte County Railroad.  The Chico & Northern was dissolved into Southern Pacific in 1912 and never operated any of the line.

Southern Pacific Transportation Company: The Southern Pacific (reporting mark SP) (or Espee from the railroad initials- SP) was a name of multiple American Class I railroads that existed from 1865 to 1998 and all operated in the Western United States.  The names that represented the Southern Pacific were Southern Pacific Railroad, Southern Pacific Company and Southern Pacific Transportation Company.

Southern Pacific Railroad of Mexico: The Southern Pacific Railroad of Mexico (reporting mark SPM) was a railroad subsidiary of the Southern Pacific Company in Mexico, operating from Nogales, Sonora, to Mazatlán, Sinaloa.  The Sonora Railway Company began construction in 1880 and opened a link to Nogales in 1882; it was a subsidiary of the American "Sante Fe" line and was subsidized by the Mexican government.  The Southern Pacific bought the Sonora Railway Company in 1898 and operated it as the "Southern Pacific of Mexico" until 1951.

Southern Pacific class AC-12: Southern Pacific Railroad's AC-12 class of 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.

Sparks, Nevada: Sparks is a city in Washoe County, Nevada, United States.  It was founded in 1904 and incorporated on March 15, 1905, and is located just east of Reno.  The 2010 U.S. Census Bureau population count was 90,264.  It is the fifth most populous city in Nevada.  It is named after the late Nevada Governor John Sparks, a member of the Silver Party.

Sierra Nevada (ferry): Sierra Nevada was a steel-hulled steam-powered passenger ferry operated on San Francisco Bay.  The ferry was built for the Western Pacific Railroad as "Edward T. Jeffery" in 1913 and subsequently renamed "Feather River".  The ferry offered connecting service to San Francisco for Western Pacific train passengers arriving in Oakland, California.  The ferry was sold to Southern Pacific Transportation Company when Western Pacific began using Southern Pacific's Oakland ferry facilities in May, 1933.  Southern Pacific renamed the ferry "Sierra Nevada" and placed it in commuter service between San Francisco and Alameda, California until that route was discontinued in 1939.  The ferry was leased to the Key System for the Golden Gate International Exposition on Treasure Island from 1939 through 1940.  In 1942, the ferry was requisitioned by the federal government to carry shipyard workers from San Francisco to Richmond Yard 1 through World War II.  The Richmond-San Rafael Ferry Company purchased the ferry in 1947 and rebuilt it to carry automobiles between Richmond and San Rafael until the Richmond–San Rafael Bridge opened on 1 September 1956.  The ferry was then towed to San Pedro, Los Angeles where she sank in 1978.

Southern Pacific 4460: Southern Pacific 4460 is the only surviving GS-6 Class steam locomotive.  The GS-6 is a semi-streamlined 4-8-4 Northern type steam locomotive.  GS stands for "Golden State".  The locomotive was built by the Lima Locomotive Works for the Southern Pacific Railroad in 1943.  The GS-6 lacked side skirting and red and orange "Daylight" paint found on previous locomotives of the GS class, and were painted black and silver instead.  This was because the US government controlled locomotive manufacturers during World War II and had turned down Southern Pacific's order of fourteen new Daylight locomotives.  Southern Pacific re-designed the engine for general service and it was finally approved, but the government took four of them and gave them to the smaller and power-starved Western Pacific Railroad.  Their smaller size when compared to previous GS class locomotives and the fact that they were built during World War II earned them the nicknames of "War Babies" and "Baby Daylights".

Southern Pacific class AC-9: The AC-9 was one of two Southern Pacific Railroad's articulated steam locomotive class that ran smokebox forward after 1920.  Twelve AC-9 class locomotives were built by Lima in 1939 and were Southern Pacific's largest and heaviest steam engines, partly a consequence of low quality coal these engines were designed to burn.  The AC-9s were partially streamlined (the only articulated steam engines to be so equipped), having "skyline casings" inspired by Lima's GS series of 4-8-4's, also made for Southern Pacific, and were equipped with coal tenders, unlike the cab forwards.  The wheel arrangement was 2-8-8-4, which was nicknamed "Yellowstone".  Between 1939 and 1952, all the twelve AC-9 engines were in service between Tucumcari, New Mexico, El Paso, Texas, and Tucson, Arizona, where they mainly pulled freight trains and occasionally also passenger trains such as the "Golden State Limited".  In 1952, they were converted to burn oil instead of coal, and moved to Southern Pacific's Modoc line between Sparks, Nevada, and Alturas, California, where they worked in freight service from 1953 until retirement in 1956.

Southern Pacific Depot: The names Southern Pacific Depot, Southern Pacific Railroad Station, Southern Pacific Railroad Depot, and variations, apply to a number of train stations operated by the Southern Pacific Railroad.  Many of these are historic and are listed on the U.S. National Register of Historic Places (NRHP).

Santa Fe Pacific Corporation: The Santa Fe Pacific Corporation was formed as the Santa Fe Southern Pacific Corporation on  23, 1983 (1983--) by the merger of Santa Fe Industries, which owned the Atchison, Topeka and Santa Fe Railway, with the Southern Pacific Company, which owned the Southern Pacific Railroad.  After the Interstate Commerce Commission denied their plan to merge their railroads as the Southern Pacific Santa Fe Railroad the holding company name was shortened and the Southern Pacific Railroad sold.

Supporting paragraphs: Southern Pacific class AC-9 Sparks, Nevada
Answer: 1952