This branch was originally constructed around 1917 by the San Pedro, Los Angeles and Salt Lake Railroad, and once was Union Pacific's entry into the citrus-growing region of Orange County. The line passed through what are now residential areas and light industrial/shopping areas. At one point the line extended as far east/south as Anaheim, but it was cut back over the years.
East of Whittier, the UP branch paralleled a Pacific Electric branch between Colima Junction and Fullerton Junction. In 1962, the UP added a connection to the PE line and abandoned its route. At the same time, the PE gained the right to use the somewhat-parallel UP between Fullerton Junction and Fullerton, and the PE abandoned that segment of its line. The UP segment between Whittier Junction and Colima Junction was abandoned a few years after the UP-SP merger in 1995, as UP trains now use the former PE line to reach the last remaining segment of UP track at Fullerton Junction.
The right-of-way is easily located and is used as an informal path. However, the Whittier Police Department has posted "No Trespassing" signs along much of the right of way, likely at the request of the homeowners who live by the old grade.
See also The Whittier Branch.