The concept is more than vague, alternate routes for re-routes of the Blue through DC have been studied at a projected traffic level by the WMATA planners. There are viewgraph presentations posted last year on a number of alternate extensions and new lines they have analyzed at the
http://planitmetro.com/ website. The alternate route for the Blue Line that gets some traction is to have go under the Rosslyn station, under the Potomac to Georgetown with 2 new stations in Georgetown, then turn east under M street, connect at Mt. Vernon Sq, then head SE to Union station providing a second Metro line at Union station, then reconnect to the current Blue line west of Stadium Armory where the Orange/Blue cross the Anacostia.