The yard at West Colton was built by Southern Pacific in 1972, at the junction of the lines west into Los Angeles and north over the Colton-Palmdale Cutoff (opened in 1968), to replace the classification yards in Los Angeles that were no longer on the main route between the Sunset Route and the San Joaquin Valley line. The yard is six miles long, and has nine reception tracks that can hold 1800 cars, 48 classification tracks (between 2400 ft and 3300 ft. long), and eleven departure tracks that can hold 2000 cars. South of the bowl are four more departure tracks, seventeen service tracks for bad order cars, and two runaround tracks. All departures leave from the west end of the yard (since both the Sunset Route and Colton-Palmdale Cutoff are in that direction), but there is a balloon track that can turns trains west if they are headed in that direction. In 1993, the yard created 15 long-distance manifests per day. Usage has expanded considerably since the UP/SP merger in 1996 and the subsequent transfer of classification work that had been performed in Yermo.
The yard is located on the south side of the Sunset Route main line, with long passing track, and on the south side of I-10, some 45 miles east of Los Angeles. A diesel locomotive maintenance facility is also located here. This vast yard is laid out along the south side of the main track. Beginning at Sierra Avenue one sees the receiving yard (where cars are inspected and checked against wheel reports), then just east of Cedar Avenue the receiving tracks lead to the hump crest and the control tower/office, followed by the “bowl” with 48 classification tracks, which narrow down to pass under Riverside Avenue. Southwest of the Riverside Avenue overpass is the locomotive facility and the engineering headquarters. East of Riverside Avenue are the departure tracks, stretching to Pepper Avenue. At the east end of the yard, Pepper Avenue crosses overhead on a bridge. East Of Pepper Avenue is a balloon track for turning trains and power.