This branch was originally constructed by a predecessor of the Southern Pacific in the 1870s. It passed through a succession of owners, eventually becoming part of the Los Angeles Pacific and then Pacific Electric in 1911. The line actually branched off the the Pacific Electric's southern four-track main at Amoco Junction (near the corner of Long Beach Boulevard and 25th Street), and headed generally west to Santa Monica. The term "air line" referred to a direct rail route before commercial air travel became popular. Part of this line ran in the median of Exposition Boulevard, but there was no true "street running" except in Santa Monica. The section of line that passed between the University of Southern California and Exposition Park was used to display other rail lines' streamliner passenger trains during the 1930s. Passenger service ended in the early 1950s, and its private right-of-way and freight customers helped the line survive several more decades.
By the 1970s, the western end of track was at Fisher Lumber in Santa Monica. The route was later cut back further to Palms and Culver City, and ultimately a short section that ran only a few blocks from Amoco Junction to a siding at Jefferson Street. Part of the last segment is still intact, but it is out-of-service and grade crossing signals have been removed.
As of 2003, the line is still easy to locate, though some areas are paved over. It passes through a heavily built-up area, including abandoned industrial facilities used by transients, active businesses, and a wide variety of neighborhoods from working class to upper middle class. The Palms depot still survives but has been moved off line. The route roughly parallels the I-10 Santa Monica Freeway, which is frequently congested. Accordingly, there have been tentative plans to rebuild and reopen the rail line as part of LA's Metro or commuter rail system. Funding constraints and fierce opposition in some neighborhoods has prevented any significant progress.
The line served Amoco Junction, Nevin, Jefferson Siding, Grand Ave. Spur, University, Cienega, Airville, Sentous, Culver Junction (site of PE crossing and later SP junction), Winslow, Palms, Winship, Home Junction (site of a junction with another abandoned railroad), and Bergamot (Santa Monica).
Update: This right-of-way is now in use by Metro, the Los Angeles metropolitan transit authority, as their Expo Rail Line. You can find more information about this line on their website.