Most trains operate along a 39-mile (63 km) stretch between Washington and Baltimore Penn, with limited service to Martin State Airport and Perryville.

It is the fastest commuter rail line in North America, with equipment capable of operating at speeds up to 125 miles per hour (201 km/h).

Meanwhile, the PRR ended B&O trackage rights over the PW&B in 1884, forcing it to open its own parallel route in 1886.

In the mid-20th century, passenger rail service declined owing to a variety of factors, particularly the advent of the automobile, even as commuting between suburban locations and urban business districts remained common.

In early 1974, the B&O threatened to discontinue its remaining unsubsidized commuter services, citing heavy losses.

[23] Prior to 1978, most ex-PRR Baltimore–Washington service was operated by aging MP54 electric multiple units, most dating back to the line's 1933 electrification.

In 1978, Amtrak and the City of Baltimore negotiated with the New Jersey Department of Transportation to lease a number of new Arrow railcars to replace the MP54s.

[24] With funding from Pennsylvania and Maryland, Amtrak used some of the cars to initiate a Philadelphia–Washington commuter trip, the Chesapeake, on April 30, 1978.

[26] Two additional round trips – one in the peak direction, and one reverse for commuters working in Baltimore – were added on July 5, 1983.

[27] On October 30, 1983, Amtrak and MARC moved from Capital Beltway into a new platform and waiting room at nearby New Carrollton station, which had been served by the Washington Metro since 1978.

[31] In 1981, MDOT began installing highway signs to point drivers to commuter rail stations.

[21] On May 1, 1991, MARC service was extended north from Baltimore to Perryville with intermediate stops at Martin State Airport, Edgewood, and Aberdeen.

[41] On December 12, 1994, Muirkirk station (originally planned as South Laurel) was opened to reduce congestion on nearby Route 1.

The collision occurred at Georgetown Junction on a snow-swept stretch of track just west of Silver Spring, Maryland.

Three died of injuries suffered in the impact alone, with the rest succumbing to the ensuing smoke and flames or a combination of the two.

The official investigation also suggests that the accident might have been prevented if a human-factors analysis had been conducted when modifications to the track signaling system were made in 1992 with the closing of nearby QN tower.

In June 2010, the MTA began looking for a new operations and maintenance contractor to replace CSX Transportation for the Camden and Brunswick lines.

[47] Controversy arose when the French-owned and Montgomery County, Maryland-based Keolis, already operating Virginia Railway Express trains, was the only bidder for the contract.

Before bidding reopened in 2011, Maryland passed a law requiring Keolis' majority owner, French state railway company SNCF,[48] to fully disclose its role in transporting Jews to concentration camps during World War II, at the request of Leo Bretholz and other Holocaust survivors.

This disclosure would need to meet the satisfaction of the Maryland state archivist before Keolis would be allowed to place a bid for MARC service.

[49] In June 2011, the future of Keolis's ability to bid on the MARC contract remained up in the air with the new disclosure law in place.

[50][51] MTA issued a new RFP for the operations and maintenance of MARC services on the Brunswick and Camden Lines on July 14, 2011, with a deadline for proposals on November 21, 2011.

[52] On October 17, 2012, the $204 million contract was awarded to the Canadian company Bombardier Transportation,[53] effectively ending the Keolis controversy.

The pre-service transition period began on the Thursday of that week, during which time CSXT continued to operate MARC trains.

In the first decade of the 21st century, MARC ridership increased significantly, and the system neared capacity for its current configuration.

With the area population growing and the BRAC process poised to bring new jobs to Aberdeen Proving Ground and Fort Meade, both near MARC stations, the state saw the need to expand service.

Though funding sources had not been established at that time, the plan represented the state's goals of increasing capacity and flexibility.

[77] On April 13, 2023, MDOT announced an agreement with the Virginia Passenger Rail Authority and the Delaware Transit Corporation for expansion possibilities beyond the current termini of MARC train service.

In the short-term, a plan to allow VRE and MARC passenger tickets to be honored on each system is under consideration.