In the spring of 1868, the Sierra Nevada were finally "conquered" by the Central Pacific Railroad (CPRR), after almost three years of sustained construction effort by mainly Chinese laborers, with the successful completion at Donner Pass of its 1,659-foot (506 m) Tunnel #6 and associated grade, thus permitting the establishment of commercial transportation "en masse" of passengers and freight over the Sierra for the first time. Following a route first surveyed and proposed by CPRR's original Chief Engineer, Theodore D. Judah (1826–1863), the construction of the four tunnels, several miles of snowsheds and two Chinese Walls necessary to breach Donner Summit constituted the most difficult engineering and construction challenge of the original Sacramento–Ogden CPRR route.