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. However, a majority of trips remain diesel powered. 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.

MARC is a commuter rail system in the Washington–Baltimore area. MARC is administered by the Maryland Transit Administration (MTA) and operated under contract by Alstom and Amtrak on track owned by CSX Transportation (CSXT) and Amtrak. In 2023, the system had a ridership of 3,860,600, or about 12,400 per weekday as of the fourth quarter of 2023, less than pre-COVID-19 pandemic weekday ridership of 40,000.