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 line, MARC's second longest at 74 miles, is operated under contract to MARC 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 & Ohio Railroad (B&O), which date to the mid-19th century.