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The Southern Pacific Railroad pushed the line down the San Joaquin Valley in 1874 and began building over the Tehachapi Mountains in 1875. Tunnels were the biggest holdup for construction. The longest of the 18 tunnels on the stretch between Bakersfield and Tehachapi was 1,156 feet and wasn't completed until March 1876. At the same time crews were building northward from Los Angeles. The longest tunnel was 6,966 feet and it wasn't opened until July 1876. The two lines met at Lang in Soledad Canyon in September 1876. The SP then had a line connecting central California with Los Angeles.
After 1946, the railroad began changing the signal system from semaphore signals to searchlight signals. Around 1955 in preparation for the construction of the Palmdale Colton Cut-off, the SP lengthened three sidings on the Mojave Desert: Ansel, Oban, and Denis to over 8,300 feet. The short sidings at Fleta, Gloster, and Rosamond were removed. To keep traffic fluid in Soledad Canyon, the siding at Ravenna was lengthened to around 6,000 feet at this time.
A major change to operations came when the Palmdale Colton Cut-off officially opened on July 11, 1967. Trains that had operated through Taylor Yard in Los Angeles and Soledad Canyon were operated through Cajon Pass. Initially, there were only about ten trains a day operated over the Cut-off. Without block signals, operations over the Cut-off were constricted. The SP finally began installing CTC starting around West Colton in 1979 and worked north. The line from Mojave all the way to the yard at West Colton was under CTC by mid-1980. As reference, I've added a timetable for the Mojave Subdivision. All photographs are on disk 48.