of its 1,659-foot (506 m) Tunnel #6 (a.k.a. the Summit Tunnel) 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 a hand-crafted stone retaining wall 75-foot tall (a.k.a. Chinese or China Wall in recognition of the Chinese builders) necessary to breach Donner Summit constituted the most difficult engineering and construction challenge of the original Sacramento–Ogden CPRR route. Principally designed and built under the personal, often on-site direction of CPRR's Chief Assistant Engineer, Lewis M. Clement (1837–1914), the original (Track 1) summit grade remained in daily use from June 18, 1868, when the first CPRR passenger train ran through the Summit Tunnel, until 1993 when the Southern Pacific Railroad (SP) (which operated the CPRR-built Oakland-Ogden line until its 1996 merger with the Union Pacific Railroad (UP)) abandoned the 6.7 mile (10.7 km) section of Track #1 over the summit running between the Norden complex (Shed 26, MP 192.1) and the covered crossovers in Shed #47 (MP 198.8), one mile east of the old flyover at Eder. All traffic has since operated over the Track #2 grade crossing the summit south of Donner Pass through the -long Tunnel #41 running under Mount Judah between Soda Springs and Eder. SP made this change because the railroad considered Track 2 and Tunnel 41 (which was opened in 1925 when the summit section of the grade was finally double tracked) to be easier and less expensive to maintain during the harsh Sierra winters than the Track 1 tunnels and snow sheds over the summit. In conjunction with major ongoing upgrades and expansions being made to the Port of Oakland in order to better accommodate the rapidly growing North American trade with Asia and the Pacific, the cooperation of UP, the Port's principal rail partner, has been sought to "construct a second track and raise tunnel clearances over Donner Pass for container trains linking California with the rest of the country." This would likely require either a new parallel tunnel next to Tunnel 41 or the replacement of the summit section of Track 1 between the Norden complex and Shed 47; either would increase capacity and effectively eliminate delays currently caused by having to run