This line was the AT&SF Harbor Subdivision, which connected the AT&SF Transcon mainline in Vernon (just west of Hobart Yard) to the Los Angeles Harbor at Wilmington. The line was constructed in stages, with the original section built in the late 1800s, and the extension from El Segundo to the harbor was built in the 1920s.
The line had numerous on-line sidings and rail-served industries; over the years most of these have closed. In the 1980s BNSF began running container freight trains from the harbor; the Harbor Subdivision was used to get the trains to the Transcon. A route that had only seen a couple of freights a day eventually had a dozen or more double stack container trains each day, running at all hours.
The heavy rail congestion on this line and others eventually led to the construction of the Alameda Corridor, a grade-separated triple track mainline running along UP's (former SP) route that runs parallel to Alameda Street. The Alamdea corridor opened in April 2002, and BNSF double stack trains were rerouted to the corridor line. The middle segment of the Harbor Sub was then taken "out of service", though it remains available in case of emergency. The line has not seen a train since 2002.
Thanks to Mike Palmer for contributing information about this route.