This bridge was actually built by Pacific Electric, which was a more-or-less independent subsidiary of Southern Pacific until absorbed into SP in 1965. It was built to reach a Columbia Steel plant, and crossed over the PE San Pedro via Gardena and Torrance Line running under the two, taller, center arches of the bridge, between the dual roadways of Torrance Blvd. under the outer, lower, arches. This was one of the few places where one Pacific Electric line crossed over another via a grade separation.
I was at this bridge today and was quite surprised to find that it is not open to pedestrian traffic. It's a puzzle. The track has been lifted from the railroad right-of-way but not the actual bridge deck. The deck is fenced with locked gates. I will post photos.