Soon, you may be able to get off the train at Gallery Place and walk the few blocks to Metro Center (and vice-versa) to change trains without paying two fares.
WMATA is considering instituting a "virtual tunnel" between the two stops, in which exiting Gallery Place and entering Metro Center would be treated just as if they were connected. That way you could switch from the Green and Yellow Lines to the Blue or Orange by doing this instead of traveling farther out of your way and transferring at a consolidated station (like L'Enfant Plaza) or paying double the fare.
WMATA is starting this system by testing it at Farragut North and Farragut West, according to the Washington Examiner, which would connect the Red Line with the Orange and Blue Lines.
Rather than building costly tunnels, technology is giving riders the same effect. While you won't be able to avoid the cold, snow, rain, heat, etc., you will save some money or time, and life may be a bit easier.