This line was originally constructed by the Pacific Electric in the early 1910s, and served the citrus groves and oil fields in northern Orange County. It crossed the AT&SF Olinda branch at grade at a station called Carlton. This PE line paralleled the south side of Imperial Highway for the easternmost three miles. Eventually the oil fields closed and the citrus groves were replaced with houses, schools, churches, businesses, etc. The line was cut back to a freight customer, Brea Chemical, during the PE days (before the SP formally merged the PE in 1965).
In the 1990s, after SP was merged into the UP, the line was cut back another mile and a half, to Brea. The line currently ends just west of State College Boulevard in Brea. In Yorba Linda, at the corner of Imperial Highway and Lemon Drive, a yellow caboose is on display along with some baggage carts. (The caboose's identifying information has been painted over.) Until 2009, a stretch of track was visible at the intersection of Imperial Highway and Rose Avenue, but that remnant has now been removed.
Thanks to Mike Palmer for contributing information about this route.