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

[8] By 1966, WMATA and Arlington County planners had agreed "to realign the rapid transit through high-density commercial-office-apartment areas in the vicinity of Wilson Boulevard instead of the freeway's median between the river and Glebe Road.

[10] In March 1968, WMATA approved its Adopted Regional System (ARS) plan that included suburban mass transit lines that followed the median of the proposed Interstate 66 through Virginia to Vienna and the CSX/Amtrak railroad right-of-way in Prince George's County, Maryland.

[11] In 1976, Robert Patricelli, federal Urban Mass Transportation Administrator, ordered Metro to conduct an alternatives analysis of the portion of its system that was not already under contract.

[13][14] Service on the Orange Line began on November 20, 1978 between National Airport and New Carrollton, with five new stations being added to the existing network from Stadium–Armory.

[11] On January 13, 1982, an Orange Line train derailed as it was being backed up from an improperly closed rail switch between the Federal Triangle and Smithsonian stations, resulting in the deaths of three passengers.

[25][26] The original plan called for trains to terminate at West Falls Church, but this was instead changed to Ballston due to effects of the ongoing COVID-19 pandemic and low ridership.

[31] Starting at its western terminus at the Vienna/Fairfax-GMU station in Virginia, the tracks run on the median strip of Interstate 66 meeting up with the Silver Line immediately after West Falls Church until they enter a tunnel under Fairfax Drive just before Ballston–MU.

[32] The tunnel continues under the Potomac River and bends to the east to travel under I Street NW in the District of Columbia towards Foggy Bottom–GWU.

The tunnel then turns south under 12th Street Northwest and enters the lower level of the Metro Center station underneath the Red Line.

[32] The elevated tracks follow Benning Road across the Anacostia River and then would go on to part ways with the Blue and Silver Lines.

After splitting with the Blue and Silver lines (which descend back underground towards Benning Road), the above ground tracks continue for the Orange Line only along DC Route 295 between Minnesota Avenue and Deanwood stations and then follow the CSX/Amtrak/MARC railroad in Prince George’s County to serve Cheverly along with Landover before reaching the eastern terminus at New Carrollton.

[34][needs update] The following stations are along the line, from west to east: The Virginia Department of Transportation (VDOT) announced on January 18, 2008 that it and the Virginia Department of Rail and Public Transportation (VDPRT) had begun work on a draft environmental impact statement (EIS) for the I-66 corridor in Fairfax and Prince William counties.