The Royal Blue was the Baltimore and Ohio Railroad (B&O)'s flagship passenger train between New York City and Washington, D.C., in the United States, beginning in 1890. The Baltimore-based B&O also used the name between 1890 and 1917 for its improved passenger service between New York and Washington, collectively dubbed the Royal Blue Line. Using variants such as the Royal Limited and Royal Special for individual Royal Blue trains, the B&O operated the service in partnership with the Reading Railroad and the Central Railroad of New Jersey. Principal intermediate cities served were Philadelphia, Wilmington, and Baltimore. Later, as Europe reeled from the carnage of World War I and connotations of European royalty fell into disfavor, the B&O discreetly omitted the sobriquet Royal Blue Line from its New York passenger service and the Royal Blue disappeared from B&O timetables. Beginning in 1917, former Royal Blue Line trains were renamed: the Royal Limited, for example, became the National Limited, continuing west from Washington to St. Louis via Cincinnati. During the Depression, the B&O hearkened back to the halcyon pre-World War I era when it launched a re-christened Royal Blue train between New York and Washington in 1935. The B&O finally discontinued all passenger service north of Baltimore on April 26, 1958, including the Royal Blue.

The Reading Company was a Philadelphia-headquartered railroad that provided passenger and freight transport in eastern Pennsylvania and neighboring states from 1924 until its acquisition by Conrail in 1976.