the corridor of modern SR 14 was also in use by the Southern Pacific Railroad. South of Mojave was used for a main line, the corridor north of Mojave follows a branch line. The main line connected Los Angeles with the Central Valley, via Soledad Pass and Tehachapi Pass. While significantly longer than the more direct Ridge Route (east of modern I-5), Tehachapi Pass is lower than Tejon Pass along the Ridge Route, with a longer, less steep grade on the descent into the Central Valley. While the Tehaachapi Pass portion of this line has remained the same, over time another route was built from Mojave, across the Antelope Valley towards Cajon Pass to cross the San Gabriel Mountains, there merging with another main rail trunk to Los Angeles. The Cajon Pass fork remains the primary freight rail line to connect southern and northern California in use today, now owned and operated by the Union Pacific Railroad. The fork paralleling SR 14 and crossing at Soledad Pass is still a contiguous line, but not used for through freight traffic as a significant portion was sold to the predecessors of the Southern California Regional Rail Authority to become the Antelope Valley Line of the Metrolink commuter rail service. The branch north of Mojave was built when the Southern Pacific acquired the unfinished Carson and Colorado Railroad in 1900. The Southern Pacific built a standard gauge connector to the narrow gauge Carson and Colorado line from their main at Mojave. Although plans were to eventually convert this acquired line to standard gauge, most of the line was abandoned before the conversion was complete. The southern portion of this line is still active and connects to the Trona Railway. The Midland Trail was one of the first organized coast-to-coast trails in the United States. In the trail's infancy, its routing changed numerous times. By 1925, the Midland Trail was established along what is modern State Route 168, joining El Camino Sierra in Big Pine. Other named trails that eventually followed this route included the Theodore Roosevelt highway, and Grand Army of the Republic Highway. Parts of modern SR 14 continue to be signed with these names, and north of Los Angeles County is still officially designated "El Camino Sierra / Midland Trail" as well as the aforementioned "Aerospace Highway". U.S. Route 6 US 6 was extended from Greeley, Colorado, to Long Beach, California,