In 1831 the Maryland General Assembly authorized the B&O to build a branch from their main line within 8 miles of Baltimore, to Washington. As this line would take much business from the parallel turnpikes, especially the Washington and Baltimore Turnpike, the charter specifically allowed those companies to subscribe to the stock of the railroad. Construction began in July 1833, and the line opened on August 25, 1835, splitting from the B&O main line at Relay, roughly 7 miles from Baltimore. Notable structural features on the original line include the Thomas Viaduct, the first multi-span masonryrailroad bridge in the United States, and the largest bridge in the country when it was completed in 1835; and the earliest example of an irontruss bridge designed by Wendel Bollman and installed at Savage.
The Alexandria Extension, originally called the Alexandria Branch, was built in 1874 and ran 12.5 miles to Shepherds Landing along the eastern shore of the Potomac River, to provide a connection to Virginia. The B&O had lost access to the Long Bridge across the river in 1870 due to political maneuvering by the Baltimore and Potomac Railroad. In order to efficiently route its trains to points south of Washington, the B&O set up a car float operation at Shepherds Landing, which carried freight cars across the river to Alexandria, Virginia. The ferry operation was discontinued in 1906 when the B&O obtained trackage rights from PRR. The B&O built a connecting track from the Alexandria Extension to the B&P's Anacostia Railroad Bridge, which provided access to PRR tracks in southwest D.C. and the Long Bridge. During World War II, the B&O re-activated the Shepherds Landing crossing at the request of the U.S. Army. The Army had requested an additional Potomac River crossing for troop movements to supplement those on the Long Bridge, and the Corps of Engineers built a temporary bridge across the river in 1942. Both B&O and PRR trains traveled over the 3,360 foot bridge. Train operations on the bridge ceased in 1945 at the end of the war, and the bridge was demolished in 1947. For many years the junction at Hyattsville, called Alexandria Junction, was controlled by JD Tower. B&O built its first interlocking tower building at the site in 1894, and rebuilt the tower in 1912 and again in 1917. CSX closed this tower in 1992, and demolished it in 1994 after a fire. The original track from the junction at Anacostia Bridge to Shepherds Landing became known as the Shepherd Branch. This spur served several industries, including Bolling Air Force Base and the Blue Plains Advanced Wastewater Treatment Plant. The Blue Plains plant was the only customer on the branch when rail service ended in 2001.
Acquisition
The Chesapeake and Ohio Railway acquired control of the B&O in 1962, and the B&O and C&O both became a subsidiaries of the Chessie System in 1973. In 1980 the Chessie System merged with the Seaboard System to form CSX Corporation. In 1987 the CSX Corporation merged the B&O into the C&O, then, later that same year, merged the C&O into CSX Transportation, which now owns the line.