Metrolink's Antelope Valley Line and the Union Pacific Railroad (formerly Southern Pacific Railroad) go through the pass via the San Fernando Tunnel. The took a year and a half to complete. Over 1,500 mostly Chinese laborers took part in the tunnel construction, which began at the south end of the mountain on March 22, 1875. Many of them had prior experience working on Southern Pacific's tunnels in the Tehachapi Pass. Due to the sandstone composition of the mountain that was saturated with water and oil, frequent cave-ins occurred and the bore had to be constantly shored up by timbers during excavation.