However, it operates parallel to CSX Transportation freight trains along the railroad's Metropolitan Subdivision from the D.C. neighborhood of Brentwood north past Silver Spring, Maryland, and continuing through Twinbrook.

[2] In 1959, the study's final report recommended two rapid transit lines which anticipated subways in downtown Washington.

[4] The National Capital Transportation Agency's 1962 Transportation in the National Capital Region report anticipated much of the present Red Line route, with the Red Line following the Baltimore and Ohio Railroad right-of-way between Silver Spring and Rockville instead of a direct route between Bethesda and Rockville.

[5] With the formation of WMATA in October 1966,[6][7] planning of the system shifted from federal hands to a regional body with representatives of the District, Maryland, and Virginia.

[10] An early proposal from 1967 was more extensive then what was ultimately approved, with the Red Line's western terminus being in Germantown instead of Shady Grove.

[12] Montgomery County officials opposed ending the Red Line in downtown Rockville, saying it would cause congestion in the area and use scarce vacant land for a storage yard.

[13] Metro decided to propose to extend the Red Line one more station to Shady Grove and the U.S. Department of Transportation conditionally approved funding for the extension on July 26, 1975.

[16] Service on the Red Line (and the Metro as a whole) began on March 27, 1976, with operation between Farragut North and Rhode Island Avenue.

[22][23] From March 26, 2020, until June 28, 2020, trains were bypassing Grosvenor–Strathmore, Cleveland Park, and Judiciary Square stations due to the 2020 coronavirus pandemic.

[citation needed] On February 25, 2022, WMATA opened a new entrance on the east side of Rockville Pike and a new elevator and staircase to the platforms at the Medical Center station, eliminating the need for thousands of daily riders who emerge from the station on the west side of the Pike to cross the busy six-lane road to reach the Walter Reed National Military Medical Center.

[27] In September 2009, Montgomery County applied for a $20 million federal grant, seeking to begin construction in 2011,[28] but the project was not approved until 2013.

[citation needed] In the summer of 2026, the Medical Center and Bethesda Metro stations will be closed due to construction of a new mezzanine that will connect to the Purple line and to Elm Street.

[32] The National Transportation Safety Board's report of July 27, 2010, blamed the crash on a faulty track circuit, part of the automatic train control system.

A curve under Lafayette Park takes the tunnel east under G Street Northwest through the Metro Center and Gallery Place stations.

[36]: 178 From Gallery Place through Judiciary Square, the line runs southeast, turning east again at D Street to reach Union Station.