Washington Branch of the Baltimore and Ohio Rail Road

The Capital Subdivision is a railroad line owned and operated by CSX Transportation in the U.S. state of Maryland and the District of Columbia. The line runs from near Baltimore, Maryland southwest to Washington, D.C. along the former Baltimore and Ohio Rail Road (B&O) Washington Branch. The subdivision's Alexandria Extension provides a connection to Virginia and points south.

The northeast end of the line is at Halethorpe, Maryland, just north of the historic Thomas Viaduct, where it meets the Baltimore Terminal Subdivision and the Old Main Line Subdivision. Its southwest end is at the yard north of Union Station, at a junction with the Metropolitan Subdivision and Amtrak's Northeast Corridor.

Between Elkridge and Laurel, the Capital Subdivision's rail alignment forms the border between Howard and Anne Arundel counties, having been built before Howard County was created from western Anne Arundel County in 1844.

MARC Train's Camden Line provides passenger service over the entire length of the main line.

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