Stereo view, late 1870s or early 1880s (possibly 1877), the south side of the San Fernando train tunnel, linking the San Fernando and Santa Clarita Valleys. Earliest known photograph.

A thousand Chinese rail workers spent the better part of a year — and an unknown number gave up their lives — digging the San Fernando Railroad Tunnel. At 6,940 feet, it was the third longest tunnel in the United States when it was completed July 14, 1876. (The tracks went in and the first engine poked its head through in August.) The tunnel, along the Southern Pacific Railroad's San Joaquin Line, linked the San Fernando Valley (at the Newhall Pass) with the Santa Clarita Valley (in Railroad Canyon), facilitating the conjoinder in September 1876 of northern California with Southern California at Lang in present-day Canyon Country.

About this image: Stereo view reads, "Watkins' New Series, Yosemite and Pacific Coast. 26 Montgomery Street, And Woodward's Garden, S.F. — South end of the San Fernando Tunnel, S.P.R.R."

"Watkins" refers to Carleton E. Watkins (Nov. 11, 1829 - June 23, 1916), who became one of the most famous early Western photographers in the 1860s when he shot some 2,000 images of Yosemite with stereoscopic and mammoth plate cameras. In 1871 he established a studio in San Francisco but lost the business and his entire photographic inventory to a creditor in 1875. "Watkins New Series" refers to the decade after his business failure when he re-photographed many of the places in California and Arizona he had photographed earlier — along with some new ones, including the San Fernando Tunnel south of Newhall and the oil works at Pico Canyon. He is known to have shot stereoscopic views of the Pico Canyon works on June 26, 1877; perhaps he photographed the San Fernando Tunnel at the same time.