This was actually two separate branches prior to 1942, when both Southern Pacific and its subsidiary Pacific Electric each had its own branch to Whittier from the south. SP abandoned much of its line in 1942, and connected the remaining section to the PE branch. The area then abandoned by SP has since been extensively built up with commercial buildings and cannot be traced except with old maps. The former PE section, from Los Nietos Junction, through Whittier, to the SP section that ended at a hospital, actually remained in place until the late 1980s. The line served packing houses in Whittier; several of the crossings had "wig-wag" signals until the end. A half-mile stub still exists at the south end of the branch (see photo). Part of the ex-PE right of way is still open, while another has houses built on it. Most of the packing houses have been replaced by a shopping center. Part of the ex-SP right of way is still in place also, but there are no traces of any sidings.
See also the The Anaheim Branch.