Overland Route (Union Pacific Railroad) in the context of "Donner Summit"

America's first transcontinental railroad (known originally as the "Pacific Railroad" and later as the "Overland Route") was a 1,911-mile (3,075 km) continuous railroad line built between 1863 and 1869 that connected the existing eastern U.S. rail network at Council Bluffs, Iowa, with the Pacific coast at the Oakland Long Wharf on San Francisco Bay. The rail line was built by three private companies over public lands provided by extensive U.S. land grants. Building was financed by both state and U.S. government subsidy bonds as well as by company-issued mortgage bonds. The Western Pacific Railroad Company built 132 miles (212 km) of track from the road's western terminus at Alameda/Oakland to Sacramento, California. The Central Pacific Railroad Company of California (CPRR) constructed 690 miles (1,110 km) east from Sacramento to Promontory Summit, Utah Territory. The Union Pacific Railroad (UPRR) built 1,085 miles (1,746 km) from the road's eastern terminus at the Missouri River settlements of Council Bluffs and Omaha, Nebraska, westward to Promontory Summit.

The railroad opened for through traffic between Sacramento and Omaha on May 10, 1869, when CPRR President Leland Stanford ceremonially tapped the gold "Last Spike" (later often referred to as the "Golden Spike") with a silver hammer at Promontory Summit. In the following six months, the last leg from Sacramento to San Francisco Bay was completed. The resulting coast-to-coast railroad connection revolutionized the settlement and economy of the American West. It brought the western states and territories into alignment with the northern Union states and made transporting passengers and goods coast-to-coast considerably quicker, safer and less expensive.

The Union Pacific Railroad Company (reporting marks UP, UPP, UPY) is a Class I freight-hauling railroad that operates 8,300 locomotives over 32,200 miles (51,800 km) routes in 23 U.S. states west of Chicago and New Orleans. Union Pacific is the second largest railroad in the United States after BNSF, with which it shares a duopoly on transcontinental freight rail lines in the Western, Midwestern and West South Central United States.

Founded in 1862, the original Union Pacific Rail Road was part of the first transcontinental railroad project, later known as the Overland Route. Over the next century, UP absorbed the Missouri Pacific Railroad, the Western Pacific Railroad, the Missouri–Kansas–Texas Railroad and the Chicago, Rock Island and Pacific Railroad. In 1995, the Union Pacific merged with Chicago and North Western Transportation Company, completing its reach into the Upper Midwest. In 1996, the company merged with Southern Pacific Transportation Company, itself a giant system that was absorbed by the Denver and Rio Grande Western Railroad.

Promontory is an area of high ground in Box Elder County, Utah, United States, 32 mi (51 km) west of Brigham City and 66 mi (106 km) northwest of Salt Lake City. Rising to an elevation of 4,902 feet (1,494 m) above sea level, it lies to the north of the Promontory Mountains and the Great Salt Lake. It is notable as the location of Promontory Summit, where the first transcontinental railroad in the United States, from Sacramento to Omaha, was officially completed on May 10, 1869. The location is sometimes confused with Promontory Point, a location further south along the southern tip of the Promontory Mountains. Both locations are significant to the Overland Route: Promontory Summit was where the original, now abandoned, alignment crossed just north of the Promontory Mountains; while Promontory Point is where the modern alignment, called the Lucin Cutoff, crosses the southern tip of the Promontory Mountains.

By the summer of 1868, the Central Pacific (CP) had completed the first rail route through the Sierra Nevada mountains, and was now moving down towards the Interior Plains and the Union Pacific (UP) line. More than 4,000 workers, of whom two thirds were Chinese, had laid more than 100 mi (160 km) of track at altitudes above 7,000 ft (2,100 m). In May 1869, the railheads of the Union Pacific and the Central Pacific railroads finally met at Promontory Summit, Utah Territory. A specially-chosen Chinese and Irish crew had taken only 12 hours to lay the final 10 mi (16 km) of track in time for the ceremony.