After Spanish Fork, the rail line joins the U.S. Route 6 corridor, and the two follow each other towards Denver. Both routes follow the Spanish Fork (river) up a grade in the Wasatch Plateau, cresting at Soldier Summit. The western approach to Soldier Summit is known for the Gilluly loops, a series of horseshoe curves that allow the railroad to crest the mountains while maintaining grade that never exceeds 2.4%, unlike the highway, which was built using an older railroad grade, that features grades in excess of 5%.[3] The railroad descends from Soldier Summit following the Price River until reaching the town of Helper, so named because in the era of steam locomotives, the railroad added or removed helper engines here for trains crossing Soldier Summit. Upon exiting the Wasatch Mountains, the train follows the southern rim of the Book Cliffs, in route serving the towns of Woodside, Green River (where the rail line crosses the Green River), Thompson Springs and Cisco. Near Cisco is where the rail line for the first time meets the Colorado River, which provides the path up the Rocky Mountains in Colorado. The rail line follows and crosses the river numerous times in Colorado while ascending the Rockies. Ruby Canyon is where the rail line reaches the state line.
Colorado
The railroad enters Colorado along the north bank of the Colorado River, following the river to the Grand Valley, passing through the heart of Grand Junction and surrounding cities along the way. The tracks continue to follow the river out of the valley, routed along Debeque Canyon, Glenwood Canyon and Gore Canyon of the Colorado River towards Granby, Colorado near the headwaters of the river. The railroad departs the main stem of the Colorado river to follow the Fraser River, one of its tributaries until reaching the crest of the Rocky Mountains which is surmounted via the Moffat Tunnel. With the decommissioning of the route over Tennessee Pass, the Moffat Tunnel is the highest point on the Union Pacific system.[4]
The eastern descent from the Moffat Tunnel towards the Front Range, where Denver resides, features 33 tunnels, leading to this portion commonly called the Tunnel District. This portion of the tracks loosely follows Colorado State Highway 72, though at points the two corridors are in different canyons and several miles apart. Even past where the tracks exit the Rocky Mountains, the grade features horseshoe curves in the final descent. The tracks approach the Denver metropolitan area from the northwest, before merging with other rail lines just north of downtown Denver.
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