Donner Pass is a high mountain pass located in the Sierra Nevada mountain range, through which emigrants first traveled to California in 1844. Today, it hosts both Interstate 80 (formerly US Route 40, the "Lincoln Highway") and Union Pacific's Roseville Subdivision.
Donner Pass is well known for two different incidents that occurred near the summit. The first, and probably most known, is the infamous Donner Party that was stranded on the pass during a snowstorm in 1846. The second, and probably remembered mostly by railfans, is when the Southern Pacific's City of San Francisco was stranded at nearby Yuba Pass for 6 days during a snowstorm in January, 1952.
There are a few portions of the Donner Pass line that have been abandoned. Tunnel Zero was abandoned during World War II; it is hidden behind vegetation. Both Yuba Gap's Main #2 and Main #1 at the summit were pulled up in 1993 when SP's president decided to pull up track on SP routes to save money and use to double track the Sunset Route. It is not known why Tunnel 40 at Troy was abandoned, but it has been located. Tunnel 15 on Main #1 was abandoned in 1913 because of double tracking, but it has yet to be located (and is thus missing on the map).
Tunnel Zero: Located at milepost 132.69 on the original Donner Pass line between the stations of Bowman and Applegate, Tunnel Zero is somewhat of an oddity. The tracks here originally passed around the hillside via a series of cuts and large trestle over a ravine known as Deep Gulch. But by 1873, engineers had decided that the direct approach was the best approach, and drove a 650-foot-long, granite-lined tunnel through the hillside to the north.
This created a unique problem, however, as several tunnels had already been completed at higher elevations, and continuing the numbering sequence at this location would disrupt the numeric order. Railroad planners ultimately decided to strike a compromise and designate the tunnel as "Number Zero" to preserve the sequence.
Originally, the tunnel was to be lined with granite brought from the town of Penryn, where the local quarry was owned by a personal friend of Central Pacific Railroad construction chief Charles Crocker. Penryn granite, however, proved to be of inferior strength, and the contract for lining the tunnel was granted to a quarry near the town of Rocklin instead. (As consolation for this slight, Penryn granite was used to build the tunnel portals.)
However, the truly unique aspect of Tunnel #0 lies not in its materials, but in its shape. Often refered to as a "horseshoe tunnel," its sides taper slightly inward near the bottom, creating a more tubular shape than the more common vertical-sided construction. It was a popular design on eastern railroads, but was exceedingly rare west of the continental divide.
Things began to change in the 1890s, when construction was started on a second main line over Donner Pass. Surveyors placed the new track just a few yards to the north and slightly below the grade of the number one main, and began construction on Tunnel # 23 at the site. The work here produced unintended consequences, however, as concussions from blasting dislodged some of the granite blocks lining nearby Tunnel #0, resulting in a partial collapse. Work crews quickly cleared the debris and rebuilt the damaged section using the relatively new construction technique of reinforced concrete.
Tunnel #0 continued to serve the railroad faithfully in its modified state until 1942, when global geopolitical events began to conspire against it. With America's entry into World War II the year before, freight traffic on American railroads soared, and one of the many items being shipped were naval landing craft. These unique boats with their shallow drafts and drop-down bow ramps were essential to the American "island-hopping" strategy. Without them, American marines would never be able to wrest control of the South Pacific from the Japanese Army.
And this is where Tunnel Zero's unique design became its Achillies heel: Its tapered sides did not provide adequate clearance for the wide boats that would soon be passing through on their way from eastern manufacturers to the ports of the Pacific Coast. Reconstruction of the tunnel, it was determined, would take months, be extremely costly, and would necessitate shutting down the number one main line for the duration of the project. Such a price was deemed unfeasible, and as an alternative solution, engineers began to eye the original right-of-way which still lay abandoned, just a few yards to the south.
Work proceeded quickly from there, with several curves being adjusted and Deep Gulch trestle being replaced with an earthen fill. By mid-summer of that year, Track #1 was once again following its original alignment, and Tunnel #0 was left to history. While attempts were made in later years at transforming this structure into a mushroom farm, such ventures proved unsuccessful, and the tunnel is now slowly being consumed by the mountain from which it was carved.