This page is connected to my Purple Line web page, which proposed (starting in 2001) a Dual Route Alignment as one of three essential transit projects needed for our region.  The others being:
 

An M Street Subway (separating the Blue and Orange lines), also depicted here, with some minor improvements to the initial concept proposed by WMATA engineers; A “Brown (or Gold) line” between Lincolnia and Baileys Crossroads, connecting to a Beltway spur of the Orange at Annandale and Masonville, VA, and extending eventially to Springfield via Ravensworth.  In the latest version depicted above, the “Gold line” is paired with the Yellow and Green lines thru parts of DC.

Regarding the latter, an Orange line spur to Annandale could be built first as an extension of the Centerville segment, via Merrifield.  The Columbia Pike and Little River Turnpike tunnels would be postponed, therefore an elevated portion of the proposed "Brown" or Gold line could initially be run as a spur of the proposed Route 7-Route 1 streetcar between Baileys South and Landmark (where most of the ridership is) on a Metro-convertible elevated track.

Please Note:  The route of any future Gold, Silver or Brown Line into town have been modified to take into account the economic feasibility of an M Street Subway, such that, thanks to the Metro Center/Tidal Basin connection, only a short section of the M Street Subway (between Florida Avenue and Mount Vernon Square, connecting to Metro Center) need be built in the near future to achieve substantial gains in throughput.  A Thomas Circle spur and a 1st Street NE spur would connect to the future Georgetown/Rosslyn junction and Stadium-Armory junction, to complete the separation of the Orange and Blue lines.  Four-fifths of the tunneling would thus be postponed.

European tunneling technology allows considerations of a new 4-track crosstown tunnel under M Street NW and eventually H Street NE, where once a system-strangling trolley might have been laid down on surface tracks, only serving to bollix up existing bus lines.

Extensions of the Gold/Brown line and Yellow Lines along the Gold Coast (i.e. separating the Green and Yellow Lines), primarily serving local stops and providing an express corridor for trains from White Oak/Konterra/Silver Spring, are a far-future consideration that should be planned for connecting the Purple Line to the proposed M Street subway in the future after other projects have been built.

This map shows the full buildout of the Metrorail system in southern Maryland.  Extensions north of Shady Grove are not shown.  Running the Silver Line as a regional express service, utilising the new corridors depicted here in the urban core, allows stations to be added to the busier outer branch of the Red Line between Germantown and Medical Center, allowing Red and Silver lines to run skip-stop service.

Red Line trains would service mostly existing stops, Gaithersburg, and a new station at White Flint Mall; Silver Line trains would service mostly new stations including two new aboveground stations along Rockville Pike at Woodmont Shopping Center and Gude Drive.  Both trains would service Shady Grove where connections could be made to a (pre-existing) Corridor Cities Transitway.

For the original 2001 proposal, Click Here to visit the Purple Line website.