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 Ellicott's Mill in Maryland. Horse-drawn cars were replaced by steam locomotives the following year.