The first transcontinental railroad in the United States was completed in 1869.
The milestone was marked on May 10, 1869, when the Union Pacific and Central Pacific railroads were joined at Promontory Summit in Utah Territory. Leland Stanford, president of the Central Pacific, drove the ceremonial “Last Spike,” linking tracks built eastward from Sacramento and westward from the Missouri River.
The railroad was a massive six-year project authorized by the Pacific Railway Act of 1862. Union Pacific crews, including many Irish immigrants and Civil War veterans, built westward, while Central Pacific workers—including thousands of Chinese laborers—laid track through the Sierra Nevada and eastward across the Great Basin.
A common technical quibble is that the 1869 ceremony joined the railroad at Promontory before every rail connection to the Pacific shoreline was finished. The western link to Alameda was completed later that year, but 1869 remains the standard and fair answer.