This branch connects with the UP (SP) Huntington Beach branch, and extends about 3-4 miles west to the US Naval Weapons Station in Seal Beach. It was built during World War II, in the early 1940s, and much of the line now travels through residential and light industrial zones. (At the time, it also connected to Pacific Electric's line between Seal Beach and Huntington Beach, but that was removed in 1950.)
There are numerous spurs within the US Naval Weapons facility, which covers several acres. These spurs serve several annumintion storage areas; they can be seen (but not photographed) from the street. The Naval facility is converting to trucks for transport.
At one point, the SP had a freight branch that extended from the Navy facility further west another mile, to the edge of Seal Beach/Long Beach. This freight branch served a Department of Water & Power plant; the branch was likely used only occasionally to haul heavy equipment.
The line was abandoned on March 31, 2010.
Thanks to Mike Palmer for contributing information about this route.