When the Pennsylvania Railroad and the Baltimore and Ohio Railroad announced in 1901 that they planned to build a new union terminal, the city had two reasons to celebrate. The decision meant that both railroads would soon remove their trackwork and terminals from the National Mall. Though changes there appeared only gradually, the consolidation of the depots allowed the creation of the Mall as it appears today. Secondly, the plan to bring all the city's railroads under one roof promised that Washington would finally have a station both large enough to handle large crowds and impressive enough to befit the city's role as the federal capital.

Union Station opened on October 27, 1907, with the arrival of a Baltimore and Ohio Railroad passenger train from Pittsburgh. The terminal quickly became the portal to the Capitol. At no time was it busier than during World War II, when as many as 200,000 people passed through in a single day.

For most of its existence, Union Station served as a hub, with service of the Baltimore and Ohio Railroad, Pennsylvania Railroad, and Southern Railway. The Richmond, Fredericksburg & Potomac Railroad provided a link to Richmond, Virginia, about 100 miles (161 km) to the south, where major north-south lines of the Atlantic Coast Line Railroad and Seaboard Air Line Railroad provided service to the Carolinas, Georgia, and Florida.