Photo by PoPville flickr user Mr. T in DC

From WMATA:

Metro will continue to advance rebuilding efforts over the weekend of December 9-11, 2011, with trains sharing the same track along portions of the Red, Blue, Orange and Green lines. The work will begin at 10 p.m. Friday and continue through system closing on Sunday night.

Green/Yellow Lines
Beginning at 10 p.m. Friday and continuing through closing Sunday, Green Line trains will share one track between U Street and Georgia Ave-Petworth to allow for rail maintenance. Green Line trains will operate at normal weekend service levels with minor delays through the work zone. Yellow Line trains will operate between Huntington and Mt. Vernon Sq. only; customers traveling to/from stations north of Mt .Vernon Sq. should use Green Line trains to complete their trip.

Red Line
Beginning at 10 p.m. Friday and continuing through closing Sunday, Red Line trains will share one track in two areas: between Van Ness and Friendship Heights to allow for rail and fastener renewal, and between Takoma and Forest Glen to allow for fiber optic cable installation and platform rehabilitation. Throughout the weekend, Red Line trains will operate between Shady Grove and Glenmont every 24 minutes. Between 9 a.m. and 9 p.m. on Saturday and Sunday, additional trains will operate between Van Ness and Takoma only, providing service every 12 minutes through the Downtown core.

Blue/Orange Lines
Beginning at 10 p.m. Friday and continuing through closing Sunday, Blue Line trains will share one track between Foggy Bottom and Arlington Cemetery and Orange Line trains will share one track between Foggy Bottom and Clarendon to allow for rail, joint and fastener renewal. Orange and Blue line trains will depart endpoint terminals every 20 minutes, resulting in trains every 10 minutes at stations served by both lines.