This branch was part of the San Diego & Arizona Eastern, which was a subsidiary of the Southern Pacific.
The city of Coronado was for many decades separated from San Diego by the San Diego Bay, though Coronado is connected to the mainland by a several-mile-long narrow strip of land called The Strand that extends south to Imperial Beach. The SD&AE connected National City to Coronado with a U-shaped branch. The right leg started at National City, the base was Imperial Beach, and the left leg extended along The Strand and ended in Coronado. At some point, the Coronado end of the SD&AE line was extended, via street running, to an adjacent Naval Base that is now referred to as NASNI (Naval Air Station — North Island).
In the late 1800s, the Del Coronado resort hotel was opened in Coronado; most of its guests in its first decades of operation arrived by train on the SD&AE.
In the early 1970s, the towering San Diego-Coronado Bridge was built over the bay between Coronado and San Diego. This bridge likely contributed to the demise of the Imperial Beach-to-Coronado segment of the line, along with the general lack of carload shipments by the US Navy. There was a salt works business in Imperial Beach that remained a freight shipper until the 1980s or 1990s, but that too is also closed.
The northern connection of this branch with the rail network in National City was severed at some point and replaced with another connection further south in Chula Vista. This Chula Vista connection is unique because it includes a diamond and wye with a light rail line (freights used the line between midnight and 5am), and a wye tail track that extends down the middle of a street.
Thanks to Mike Palmer for contributing information about this route.