The California Southern Railroad was a subsidiary railroad of the Atchison, Topeka and Santa Fe Railway (Santa Fe) in Southern California. It was organized July 10 1880, and chartered on October 23 1880, to build a rail connection between what has become the city of Barstow and San Diego, California.

Construction began in National City, just south of San Diego, in 1881, and proceeded northward to the present day city of Oceanside. From there, the line turned to the northeast through Temecula Canyon, then on to the present cities of Lake Elsinore, Perris and Riverside before a connection to the Southern Pacific Railroad (SP) in Colton. Following a frog war where the SP refused to let the California Southern cross its tracks, a dispute that was resolved by court order in favor of the California Southern, construction continued northward through Cajon Pass to the present day cities of Victorville and Barstow. The line, completed on November 9 1885, formed the western end of Santa Fe's transcontinental railroad connection to Chicago. Portions of the original line are still in use today as some of the busiest rail freight and passenger routes in the United States.