I am looking at several old pictures from the interlockings along the Long Beach line. But, not that specific picture.

From some of these, it appears that it may be a guide rail from a derail design at an interlocking. This third rail was part of early derails at the Dominguez interlocking from very old-pre war pictures. I believe this design was used at other interlockings like Slauson and Amoco. The center rail appears to be to keep the derailed car from drifting too far over.