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 from Sacramento to Omaha in the United States 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 is where the original, abandoned alignment crossed the Promontory Mountains while the modern alignment, called the Lucin Cutoff, crosses the mountains at Promontory Point.

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.



North 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 constructed 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 US land grants. Construction was financed by both state and US 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 first transcontinental rail passengers arrived at the Pacific Railroad's original western terminus at the Alameda Terminal on September 6, 1869, where they transferred to the steamer Alameda for transport across the Bay to San Francisco. The road's rail terminus was moved two months later to the Oakland Long Wharf, about a mile to the north, when its expansion was completed and opened for passengers on November 8, 1869. Service between San Francisco and Oakland Pier continued to be provided by ferry.

The CPRR eventually purchased 53 miles (85 km) of UPRR-built grade from Promontory Summit (MP 828) to Ogden, Utah Territory (MP 881), which became the interchange point between trains of the two roads. The transcontinental line became popularly known as the Overland Route after the name of the principal passenger rail service to Chicago that operated over the length of the line until 1962.