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.

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.

"[1][2] Between the 1890s and World War I, the B&O's six daily Royal Blue trains providing service between New York and Washington were noted for their luxury, elegant appearance, and speed.

The car interiors were paneled in mahogany, had fully enclosed vestibules (instead of open platforms, still widely in use at the time on U.S. railroads), then-modern heating and lighting, and leaded glass windows.

[6] The project also included the construction of B&O's second passenger terminal in Baltimore, Mount Royal Station, at the north end of the Howard Street tunnel in the fashionable Bolton Hill neighborhood.

Designed by Baltimore architect E. Francis Baldwin in a blend of modified Romanesque and Renaissance styling, the station was built of Maryland granite trimmed with Indiana limestone, with a red tile roof and landmark 150-foot (46 m) clocktower.

Powered by 4-6-0 steam locomotives having exceptionally large 78-inch (198 cm) diameter driving wheels for speed, the Royal Blue trains occasionally reached 90 mph (145 km/h).

These buses were ferried across the Hudson River into Manhattan and Brooklyn, where they proceeded to various "stations" around the city on four different routes, including the Vanderbilt Hotel, Wanamaker's, Columbus Circle, and Rockefeller Center.

Connected to Grand Central Terminal by an underground concourse, it was trimmed in marble and furnished with Art Deco lighting fixtures and leather sofas.

[21] The B&O's manager of dining car services said his department's objective was "...to be hospitable to our patrons in all respects – to make them feel the comfort, convenience and homelike atmosphere of our accommodations as soon as they step on our trains.

Second, the Pennsylvania's move in the early 1930s to replace steam power with modern, smokeless electric service along its entire New York–Washington mainline was met with enthusiastic public approval.

Designated # 50 by the B&O, this marked the first single-unit, passenger road diesel locomotive use in the U.S.[7][23][24][25] Previously, early experiments with internal combustion engines to replace steam in railroad applications included short, articulated trainsets (such as Burlington's Pioneer Zephyr and Union Pacific's M-10000), double-head sets of box-cab locomotives (developed by EMC) used to power the 1936 version of the AT&SF (Santa Fe) Super Chief (similar to number 50), and the cab/booster unit combinations developed with Union Pacific's M-10002 and M-10003 – M-10006 trainsets.

[32][33] President of the United States Franklin D. Roosevelt was a frequent passenger on the B&O's Royal Blue Line during his time in office (1933–1945), when he traveled between Washington and his family home in Hyde Park, New York.

[2][24][37] Amidst the downward trend, the Royal Blue Line briefly recaptured the regal splendor of its early years on October 21, 1957, when Queen Elizabeth II and Prince Philip travelled on the B&O from Washington to New York.

As financial losses mounted, the B&O finally ceded the New York–Washington market to the Pennsylvania Railroad altogether, discontinuing all passenger service north of Baltimore on Saturday, April 26, 1958, and bringing the venerable Royal Blue to an end.

[39] As the engineer was about to ease the locomotive's throttle open for the Royal Blue's final departure from Washington Union Station at 3:45 p.m., the event was covered in a trainside remote broadcast by Edward R. Murrow on a CBS network See It Now television special.

[37] The New York Times, in a front page article accompanied by a photograph of train engineer Michael Goodnight bidding farewell to a 7-year old passenger, said "It was a sad and simple story yesterday as the nation's oldest railroad discontinued its crack Royal Blue and its five other passenger trains ... end[ing] sixty-eight years of continuous through service, operated in a gentlemanly fashion ... a kind of ante-bellum, gracious way of life ... and the reputation for very special service.

[12] The building and trainshed were subsequently acquired by the Maryland Institute College of Art (MICA) in 1964 and are preserved as examples of late 19th century industrial architecture.

[43] From 1935 to the end of service in 1958, the Royal Blue made a daily round trip, departing New York in the morning and returning from Washington in the evening.