The Brunswick Line is a MARC commuter rail line between Washington, D.C., and Martinsburg, West Virginia, with a branch to Frederick, Maryland. It primarily serves the northern and western suburbs of Washington. The 74-mile line, MARC's second-longest line, is operated by Alstom and runs on CSX-owned track, including the Metropolitan, Old Main Line, and Cumberland subdivisions. It is the successor to commuter services provided by the Baltimore and Ohio Railroad (B&O) that date to the mid-19th century.