Old Main Line Subdivision

The Old Main Line Subdivision is a railroad line owned and operated by CSX Transportation in the U.S. state of Maryland. The line runs from Relay (outside Baltimore) west to Point of Rocks, and was once the main line of the Baltimore and Ohio Railroad, one of the oldest rail lines in the United States. At its east end, it has junctions with the Capital Subdivision and the Baltimore Terminal Subdivision; its west end has a junction with the Metropolitan Subdivision.

The initial route of the Baltimore and Ohio Railroad (B&O) followed the Patapsco River valley west out of Baltimore, with the first section (to what is now Ellicott City, Maryland) opening for service in 1830. The line left the valley to cross Parr's Ridge, which, after an abortive attempt to use a system of inclined planes, was crossed via a more round-about routing through Mount Airy. It continued west to Harper's Ferry, West Virginia, passing south of Frederick on the way. This line was the only route west out of Baltimore until the Metropolitan Branch was constructed from Washington, DC to Point of Rocks in the 1870s. The section of the original route from Relay (where the Washington Branch began) to Point of Rocks became known as the "Old Main Line" (or "OML" for short) as it was relegated to secondary status, and continues to be known as the Old Main Line Subdivision in CSX timetables.

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