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This project is a 20–mile-long freight rail connector between the rail yards of downtown Los Angeles and the ports of Los Angeles and Long Beach. The 10-mile-long Mid-Corridor Trench segment, designed and built by the Tutor-Saliba Team, is the heart of the entire Corridor. The trench is an extremely large structure; the typical section is approximately 30 feet deep and 50 feet wide. It is watertight and includes two water treatment and pumping stations. The available right-of-way was so narrow at the north end that the adjacent roadways were cantilevered over the trench. Project design was complicated by the requirement for a 100-year useful life and the requirement to meet current UPRR and BNSF railway standards.

The project grade separated the entire 10-mile length by depressing three freight rail lines into a single trench. Although the trench followed an existing rail corridor, that corridor was narrow, bound on both sides by major arterial streets, and surrounded by a dense, active urban area. Major arterial streets plus two freight railroads cross the trench on 29 bridges (up to 250 feet wide). The trench passes through six cities and the county. The design required local agency approval of their portion of the trench, and of the respective adjacent improvements. Those improvements entailed the complete rebuilding of Alameda Street West for the entire length of the project, plus the partial rebuilding of Alameda Street East. Over 100 lane-miles of roadway were built.