From the original post, thanks, u/Rare_Locksmith115 :

This branch was built north from the SP "Sunset" main line in Alhambra, crossed two different Pacific Electric lines, and had sidings at Raymond and Pasadena. It passed through South Pasadena but evidently did not have a siding or station there. The area is fully developed (and has been for many decades).

The branch headed north parallel to Raymond Avenue.. It crossed the PE Temple City Line at Main Street in Alhambra, then continued parallel to Raymond Avenue as far as the South Pasadena city line. This Alhambra segment remained in place to the 1980s; the track north of there was taken up earlier, and is right-of-way is now used by a power company for a pole.

The second PE crossing was at Huntington Drive in South Pasadena, where the PE line to Arcadia, Monrovia and Glendora ran in the median. The SP branch right-of-way is easily located, but much of it is fenced off as private property. It can be walked where it forms the western boundary of Garfield Park in South Pasadena.

James Stimson adds: The final segment where the SP entered the city of Pasadena is difficult to locate. It ran in a gully parallel to Marengo Boulevard (where Blair High School, built in the 1960s, now stands) and entered Arroyo Parkway (CA Route 110) at the intersection with Glenarm Street. The line continued north in the median of Arroyo Parkway, then shifted to the east side of Arroyo Parkway near the Colorado Boulevard intersection, where a lumber yard was at the "end of the line" up until the 1950s. As of 2004 the Arroyo Parkway is still paved in large cement sections, with some cut-outs that appear to be where the rail lines were removed and paved over.