Tunnel 27 is an 846-foot, single track railroad tunnel, with granite ashlar portal faces and wingwalls. As-built, the tunnel was concrete-lined for the first fifty feet in from each portal, with the remainder lined in redwood timber; the railroad has subsequently covered the timbering with shotcrete. The tunnel is on a two-degree left-hand curve, and carries the tracks of the Union Pacific Railroad's (formerly Southern Pacific) Donner Pass line. In 1965 the east end of the tunnel collapsed, with the result that approximately 115 feet of the east end was "daylighted" and the original east portal and concreted end were left in place, free-standing. Tunnel 27 was recorded for this reason, because it shows to good effect the original construction of these tunnels. While the railroad continues to consider Tunnel 27 as an 846-foot structure, portal-to- portal, the actual current length subsurface is approximately 731 feet.
-- Historic American Engineering Record