It's added on when you download Broken Steel, it's part of the main storyline. Ignore for now kiddies. :3
From "The Vault":
The Presidential Metro is a network of metro tunnels that is added to Fallout 3 in the Broken Steel add-on.
It connects the Presidential Sub Level beneath the White House with Adams Air Force Base and The Capitol Building's East Wing. Protected and maintained by a computer system called M.A.R.Go.T., the area is guarded by Sentry Bots and swarming with ghouls.