Promontory Summit, Utah, is a historic site located in Box Elder County. It is best known as the location where the first transcontinental railroad was completed on May 10, 1869. This event marked a significant milestone in American history, connecting the eastern and western United States. The site features the famous Golden Spike, which symbolizes the completion of the railroad. Today, Promontory Summit is part of the Golden Spike National Historical Park, where visitors can learn about the impact of the railroad on the nation and see reenactments of the historic ceremony.