Penn Line

The Penn Line is a MARC passenger rail service operating between Union Station in Washington, D.C., and Perryville, Maryland, along the far southern leg of the Northeast Corridor; most trains terminate at Baltimore's Penn Station. It is MARC's only electrified line. With trains operating at speeds of up to 125 miles per hour (201 km/h), it is the fastest commuter rail line in the United States. The service is operated by Amtrak under contract to the Maryland Transit Administration. MARC sets the schedules, owns most of the stations, and controls fares, while Amtrak owns and maintains the right-of-way, supplies employees to operate trains, and maintains the rolling stock. It is the busiest of MARC's three lines, with twice as many trains and ridership as the Brunswick and Camden lines combined.

The Penn Line is the successor to commuter services between Washington and Baltimore provided by the Pennsylvania Railroad (PRR), Penn Central, and Conrail dating back as early as 1881. Additionally, Amtrak operated a commuter service named the Chesapeake between 1978 and 1983.

In 1983, Maryland, along with a number of other Northeastern states, took control of its commuter railroads. Amtrak, which had acquired the right-of-way from Penn Central, took over operation of the former PRR commuter line, which was rebranded as AMDOT (Amtrak/Maryland Department of Transportation). The Chesapeake was discontinued later in 1983 due to low ridership and redundancy with AMDOT. A year later, all commuter service in Maryland was merged under the MARC brand.

With frequent MARC and Amtrak service, the Washington-Baltimore section of the Northeast Corridor is one of the busiest rail lines in the United States.