Baltimore and Ohio Railroad

The Baltimore and Ohio Railroad (reporting marks BO, B&O) 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.

Founded to serve merchants from Baltimore who wanted to do business with settlers crossing the Appalachian Mountains, the railroad competed with several existing and proposed turnpikes and canals, including the Erie and Chesapeake and Ohio Canal. The railroad began operation in 1830 on a 13-mile line between Baltimore and Elliot's Mill in Maryland. Horse-drawn cars were replaced by steam locomotives the following year.

Over the following decades, construction continued westward. During the American Civil War, the railroad sustained much damage but proved crucial to the Union victory. After the war, the B&O consolidated several feeder lines in Virginia and West Virginia, and expanded westward into Ohio, Indiana, and Illinois.

In 1962, the Chesapeake and Ohio Railroad gained control of the B&O, though it continued to operate separately. By 1970, the B&O operated 4,535 miles (7,300 km) of mainline track, plus the Staten Island Rapid Transit system and the Reading Railroad and its subsidiaries. The B&O ended long-distance passenger service in 1971, although it continued limited commuter service at Washington, D.C., and Pittsburgh. In 1987, the B&O was formally merged into the C&O, which was by then a subsidiary of CSX Transportation (CSX).

The B&O is noted as a pioneer in railroading. It was the first U.S. railroad to operate a steam locomotive, it built historic infrastructure, and it operated prestigious passenger trains. It also gained fame as one of the four railroads in the original version of the board game Monopoly.