The City of Denver was a streamlined passenger train operated by the Union Pacific Railroad between Chicago, Illinois, and Denver, Colorado. It operated between 1936 and 1971. From 1936–1955 the Chicago and North Western Railway handled the train east of Omaha, Nebraska; the Chicago, Milwaukee, St. Paul and Pacific Railroad handled it thereafter. The train was the fastest long-distance train in the United States when it debuted in 1936, covering 1,048 miles (1,687 km) in 16 hours. For almost its entire career its principal competitor was the Chicago, Burlington and Quincy Railroad's Denver Zephyr. When Amtrak assumed operation of most intercity trains in the United States in 1971, it discontinued the City of Denver, preferring to use the Burlington's route between Chicago and Denver.

A streamliner is a vehicle incorporating streamlining in a shape providing reduced air resistance. The term is applied to high-speed railway trainsets of the 1930s to 1950s, and to their successor "bullet trains". Less commonly, the term is applied to fully faired upright and recumbent bicycles. As part of the Streamline Moderne trend, the term was applied to passenger cars, trucks, and other types of light-, medium-, or heavy-duty vehicles, but now vehicle streamlining is so prevalent that it is not an outstanding characteristic. In land speed racing, it is a term applied to the long, slender, custom built, high-speed vehicles with enclosed wheels.