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.