A Penn Line train at Odenton station
|Locale||Washington, D.C. and Maryland suburbs east; Baltimore, MD and suburbs northeast|
Washington, D.C. Union Station|
|Daily ridership||24,247 |
|Operator(s)||Amtrak/ Maryland Transit Administration|
|Line length||77 mi (124 km)|
|Track gauge||1,435 mm (4 ft 8 1⁄2 in) standard gauge|
|Operating speed||125 mph (201 km/h)|
The Penn Line is a MARC commuter rail service running from Union Station in Washington, D.C. to Perryville, Maryland via Baltimore's Penn Station on Amtrak's Northeast Corridor. It is MARC's busiest and only electric line. Currently the line is the fastest commuter rail line in the country, with trains running at speeds of up to 125 miles per hour (201 km/h).  The service is operated under contract by Amtrak which supplies employees to operate trains, and maintains the right-of-way and MARC's electric locomotives and passenger cars. The line is administered by MARC, a service of the Maryland Transit Administration (MTA).
The Penn Line is the successor to commuter services provided by the Pennsylvania Railroad (PRR), Penn Central, and Conrail as long ago as the mid-19th century. Additionally, Amtrak operated a commuter service named the Chesapeake between 1978–1983.  In 1983, Maryland, along with a number of other Northeastern states, took control of its commuter railroads and the "MARC" (Maryland Area Regional Commuter) service name was established.  The Penn Line became the replacement for Amtrak's Chesapeake as well as the minimal former PRR commuter service between Washington and Baltimore.
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The Penn Line uses diesel as well as electric locomotives for powering trains. Most electric trains are 6-8 cars long (usually made up of all or mostly Kawasaki bilevels). Rush hour diesel trains are usually 6-8 cars long (usually made up of all or mostly Kawasaki bilevels). During the day, shorter 4-6 car MultiLevels or single level diesel trains from the Brunswick and Camden lines are used on the Penn Line. All weekend Penn Line trains also include a single-level Bike Car that is specially equipped to accommodate bicyclists.
All of the stations from Washington Union Station up to Halethorpe have high-level platforms, and all of the subsequent stations from West Baltimore up to Perryville, with the exception of Penn Station, have low-level platforms. This precludes the use of MARC's ex- Metra low-level boarding gallery railroad cars on the Penn Line.
The first revenue test of MARC's new Siemens Charger SC-44 diesel-electric locomotives on the Penn Line will occur during the week of April 16, 2018. During this phase, a current MPI MP36PH-3C locomotive will be paired with it for redundancy. These new locomotives "...will be prioritized for assignment to seven and eight car rush hour trains – especially those operating north of Baltimore -- as they are released for solo revenue service. The higher horsepower and higher maximum speed of the Chargers will help with on time performance and reduce occurrences of MARC trains having to stop and wait for Amtrak trains to pass." 
The Penn Line runs 57 trains during a normal weekday. A majority of these trains (39 each day) originate/terminate between Union Station in Washington and Penn Station in Baltimore. An additional 11 trains originate/terminate between Union Station and Perryville, MD, and another 5 originate/terminate between Union Station and Martin Airport. A single morning train and a single evening train runs between Perryville and Penn Station.  Unlike MARC's other two lines, the Penn Line operates all throughout the day and well into the night.
On December 7, 2013 the Penn Line also began offering limited weekend service.  Penn Line weekend service consists of 9 round trips on Saturday and 6 round trips on Sunday—primarily between Penn Station and Union Station, but with some runs extending from/to Martin State Airport.  All Penn Line weekend trains include a separate, specially-designed Bike Car which is designed to accommodate bicyclists. Bike Cars are reconditioned Sumitomo/Nippon Sharyo MARC IIA single-level commuter rail cars. One side of each car's interior is lined with bicycle racks which are arranged to secure 23 full-sized, non-collapsible bicycles, and the other side provides seating for 40 passengers.  There is no extra charge for using the Bike Car and the bicycle racks on the Bike Car are available on a first-come, first-served basis. The first Bike Car was added to select weekend trains on December 13, 2014.  The Bike Car program was expanded in 2015, and since October 31, 2015 all Penn Line weekend trains have offered a Bike Car. 
Amtrak's Acela Express, Northeast Regional, and long distance trains share tracks along the whole of the Penn Line, and share stations at Aberdeen, Penn Station, BWI Airport, New Carrollton, and Union Station. Currently, MARC passengers with monthly and weekly tickets can ride select Amtrak Northeast Regional trains during the week only.  Connections are also available to the Washington Metro's Orange Line at New Carrollton, Red Line at Washington Union Station, and to the MTA Light Rail at Baltimore Penn Station. 
The MTA has plans to extend the Penn Line to Newark station in Delaware to connect with the Wilmington/Newark Line of SEPTA or even further north to 30th Street Station in Philadelphia. Currently, the MTA funds a local bus connection between Newark and Baltimore with a transfer at Elkton station.  Longer-term plans include construction of new track and extending the line past Washington Union Station to L'Enfant Plaza station and into northern Virginia.  The planned Purple Line that will connect all three MARC lines will connect with the Penn Line at New Carrollton.
The following stations are served by Penn Line trains; not all trains stop at all stations.
- "MTA Average Weekday Ridership - by Month". Maryland Open Data Portal. June 2017. Retrieved April 16, 2018.
- Matt Van Hattem (June 30, 2006). "Maryland Rail Commuter (MARC)". Trains Magazine. Retrieved February 16, 2010.
- "1979 Amtrak Chesapeake timetable".
- "MARC History". MTA. Archived from the original on January 17, 2010.
- "Penn Line schedule change: April 23" (Press release). MTA Maryland. Retrieved April 15, 2018.
- "MARC Penn Line Weekday Schedule" (PDF). MTA Maryland. August 2018. Retrieved September 17, 2018.
- Sherman, Natalie; Wenger, Yvonne (December 7, 2013). "MARC train weekend service begins". The Baltimore Sun. Retrieved December 6, 2015.
- "MARC Penn Line Weekend Schedule" (PDF). MTA Maryland. August 2018. Retrieved September 17, 2018.
- "MTA Rolls Out Bike Cars on All Weekend Trains" (Press release). MTA Maryland. October 29, 2015. Retrieved December 6, 2015.
- Rector, Kevin (December 11, 2014). "MTA to introduce bike car to weekend MARC service". The Baltimore Sun. Retrieved December 6, 2015.
- "MARC Riders Advisory Council Meeting Summary Minutes" (PDF). MTA Maryland. January 18, 2018. Retrieved April 16, 2018.