This segment was an extension of a Union Pacific branch into the citrus-growing region of Orange County. It was built in the early 1920s, well after the Pacific Electric, Southern Pacific and Santa Fe had already constructed tracks in the area. The abandoned segment of the line paralleled the Santa Fe for its entire length. In some places the lines were adjacent; in other areas they were about a block apart.
The UP crossed the Santa Fe (now the BNSF Transcon) on a diamond at 'Basta' in west Fullerton. When the line south of Fullerton was abandoned, in the early 1990s, the diamond was removed and the UP was connected to the BNSF with a switch.
Thanks to Mike Palmer for contributing information about this route.