This route was originally a Pacific Electric line; PE was a subsidiary of SP and was merged into SP in 1965. It ran in the median of Magnolia Avenue, a major arterial street that runs southwest from Riverside to Corona.
This line crossed different lines of the AT&SF at two locations. The AT&SF mainline (now called the "Transcon") was crossed in the median of Magnolia Avenue, near Buchanan Ave. Separately, the AT&SF The Lake Elsinore Branch was crossed at Porphyry, also in the median of Magnolia Ave.
Most traces of this line are gone; much of the Magnolia Ave. median east of Porphyry has now been landscaped, and all former grade crossings have long been repaved and reconstructed. Near Porphyry the median is still mostly dirt.
Thanks to Mike Palmer for contributing information about this route.