Vince wrote that "The DC Metro has bus and subway service from BWI into D.C." This is convenient if you can maneuver your luggage fairly easily onto the bus and into the Greenbelt Metro station when you get there. Escalators and/or elevators aren't always working. A word of caution about using the shuttle to the Amtrak Station. If it is a weekday, you can take the MARC commuter train into DC. I think the fare is under $10. The MARC only runs on weekdays. Amtrak from BWI to DC can cost as much as $25 to Union Station. So judge the situation accordingly.

Gene