Final answer:
The first Transcontinental Railroad was completed in Utah, at Promontory Summit, where the Central Pacific and Union Pacific Railroads united on May 10, 1869. This was a landmark event in U.S. history, enabling faster travel and economic expansion across the country.
Step-by-step explanation:
The state where the first Transcontinental Railroad was completed at Promontory Summit, near the Great Salt Lake, linking the Central Pacific and Union Pacific Railroads is Utah. This momentous event occurred on May 10, 1869, when the rail lines were connected by the driving of the Golden Spike. The completion of this railroad had a profound impact on the United States, significantly reducing the time required to travel across the country and facilitating economic growth and the settlement of the western territories.
The Pacific Railway Act was instrumental in the development and financing of this project, providing land grants and other incentives to the railroad companies. The Union Pacific Railroad built track west from Omaha, Nebraska, while the Central Pacific Railroad moved east from Sacramento, California. Eventually, both companies joined their tracks at Promontory, Utah, marking the completion of the first transcontinental railroad.