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.