The inaugural run of the Baltimore and Ohio Railroad's Royal Blue from Washington, D.C., to New York City, the first U.S. passenger train to use electric locomotives.

The Baltimore and Ohio Railroad was the oldest railroad in the United States and the first steam-operated common carrier. Construction of the line began in 1828, and it operated as B&O from 1830 until 1987, when it was merged into the Chessie System. Its lines are today controlled by CSX Transportation.

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.

An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a battery or a supercapacitor. Locomotives with on-board fuelled prime movers, such as diesel engines or gas turbines, are classed as diesel–electric or gas turbine–electric and not as electric locomotives, because the electric generator/motor combination serves only as a power transmission system.