Your information is right about the era and the immigrant workforce, but that tunnel is hardly a secret: More than 20 Metrolink trains packed with commuters barrel through it every day. Part of the Southern Pacific Railroad, the tunnel was completed in 1876 by hundreds of workers from Guangdong Province, who flocked to Newhall and, using picks and shovels, chipped an 8,000-foot-long hole into the Santa Susana mountain range.