I have been wondering about this for a long time. Metro is considering several improvements to the current subway system, including express service trains and splitting the Blue line into two routes. Check out the WMATA press release here. I wonder a lot about the express service, since the current system only has two sets of tracks (one going in each direction). The express train wouldn't be able to skip stations on the side that it is normally on (right side), so it would obviously have to jump to the opposite side and jump back over before a train runs head first into it. This would require a lot of time coordinating, but I think this is the possibility that they are considering.