A cheaper (?) alternative to the separated Yellow line, which has something in common with Daniel's proposal, would be to build another Potomac crossing between the Pentagon (there's a switch and a stub tunnel just south of the station) and the DC waterfront. Cross the Green line at Waterfront, cross Orange and Blue at Federal Center SW, then take two lanes from the 395 spur to run up to New York Ave (maybe put a station at Mass. Ave), then a very short tunnel along M St to abut the Convention Center station.
One could run the Silver line from Dulles to K route to Courthouse to C route to Pentagon and then up this new route.
Existing Orange and Blue traffic run through the existing C-K junction.
