• Snow sheds in the Sierra Nevada Mountains during construction of the Central Pacific Railroad, 1865. Courtesy of the Utah State Historical Society via the Mountain West Digital Library.

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    Central Pacific RR - Snow Sheds P.1; 00559
    • Date
    • 1865
    • Creator
    • Russell, Andrew J
    • Description
    • Snow sheds in the Sierra Nevada Mountains overlooking Donner Lake. Photographer: Andrew J. Russell, official photographer of Union Pacific Railroad.
    • Rights
    • Digital Image © 2014 Utah State Historical Society. All Rights Reserved.
    • Partner
    • Mountain West Digital Library
    • Contributing Institution
    • Utah State Historical Society

  • An interior view of the Central Pacific Railroad's unfinished Summit Tunnel on Donner Pass. Courtesy of the Denver Public Library.

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    Summit Tunnel, before completion, Western Summit - altitude 7.042 feet
    • Date
    • ca. 1860-1870
    • Creator
    • Hart, Alfred A, 1816-1908
    • Description
    • Interior view of the west end of the Central Pacific Railroad's unfinished Summit Tunnel on Donner Pass.
    • Rights
    • Restrictions applying to use or reproduction of this image available from the Western History/Genealogy Dept., Denver Public Library.
    • Partner
    • Miss Louise Stegner- Omaha, Neb. 1.10 (1950)

  • Chinese laborers working on the Grizzly Hill Tunnel, late 1890s. Courtesy of the Utah State Historical Society via the Mountain West Digital Library.

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    Central Pacific R.R.--Construction P.11; 00529
    • Date
    • 1863
    • Creator
    • Hart, Alfred A
    • Description
    • Chinese laborers / workers fill from one-horse carts at Horse Ravine Wall near Grizzly Hill Tunnel near Shady Run in the High Sierra, 77 miles from the start of construction in Sacramento. Photograph taken by official Central Pacific photographer: Al... more
      Chinese laborers / workers fill from one-horse carts at Horse Ravine Wall near Grizzly Hill Tunnel near Shady Run in the High Sierra, 77 miles from the start of construction in Sacramento. Photograph taken by official Central Pacific photographer: Alfred A. Hart of Sacramento. Gift: Southern Pacific Photo X145. less
    • Rights
    • Digital Image © 2014 Utah State Historical Society. All Rights Reserved.
    • Partner
    • Mountain West Digital Library
    • Contributing Institution
    • Utah State Historical Society

  • A man with stands near the unfinished rail bed at the Central Pacific Railroad Summit Tunnel on Donner Pass. Courtesy of the Denver Public Library.

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    East portal Summit Tunnel, length 1,660 ft.
    • Date
    • ca. 1865-1869
    • Creator
    • Hart, Alfred A, 1816-1908
    • Description
    • View of the Central Pacific Railroad Summit Tunnel on Donner Pass; a man poses with a horse and buggy on the unfinished rail bed.
    • Rights
    • Restrictions applying to use or reproduction of this image available from the Western History/Genealogy Dept., Denver Public Library.
    • Partner
    • Miss Louise Stegner- Omaha, Neb. 1.10 (1950)

A route through the Donner Pass and Dutch Flats was identified as the safest and easiest route for the Central Pacific Railroad to traverse the Sierra Nevadas. But railroad crews had to find their own way through the mountains when going over or around them was impossible. Over a dozen tunnels were constructed to allow the Central Pacific to pass through the Sierras, requiring thousands of laborers and a great deal of engineering ingenuity.

One of the most challenging tunnels built by the Central Pacific Railroad was Tunnel No. 6, the Summit Tunnel, which ran through the top of the Donner Pass. Early snows defeated the first attempt to build it. While construction crews began digging at both the east and west ends, two additional crews dug down from above the tunnel’s route to pull out the debris through vertical shafts. The measurements used to plan the passage of the Summit Tunnel were so accurate that, having drilled through 1,659 feet of rock, the teams were only two inches off when they finally connected the eastern and western portals.

Another technological advancement that aided the completion of the Summit Tunnel was nitroglycerin. Railroad engineers used this explosive compound mixed on site to move large sections of rock and earth, which was quicker than digging by hand. The unstable nature of nitroglycerin continued to be a problem during the course of Summit Tunnel construction, causing an untold number of laborer deaths. As a result, it was not used on the remainder of the Central Pacific Line.