I decided to follow the route with Google Maps to see how much has been double-tracked. If the recent San Diego Trolley line construction is any guide, then Google is much more up-to-date than Bing or MapQuest. It's all double-track in San Diego, from the Santa Fe Depot to the big hill that it detours eastward around.
The line becomes single-track at Miramar Rd. and stays that way until it gets close to Carroll Canyon Rd. The right-of-way looks wide enough for two tracks, however.
It becomes double-track there, and and becomes single-track a bit south of the intersection of Sorrento Valley Rd. and Carmel Mountain Rd. The right-of-way becomes only a single track wide, and stays that way all the way to Solana Beach. It becomes double-track a bit south of a pedestrian overpass at Dahlia Dr., all the way to Encinitas, where it becomes single-track a bit south of E St. The right-of-way is wide enough for two tracks, however, until La Costa Ave.
Under Avenida Encinas, it becomes double-track, and in Carlsbad, it crosses a river on a double-track bridge with a single-track bridge nearby. Oceanside station is a stop for Coaster, Metrolink, Amtrak, Sprinter (Oceanside - Escondido DMU LRT), Greyhound, and plenty of local buses.
Most of the single-track parts of this line are in rights-of-way that can easily accommodate double tracks. Some of the single-track-only ROW goes in undeveloped land near rivers, and in single-track bridges across those rivers. It should be easy to widen those ROW's to double-track width add extra bridges alongside the existing ones.