You can do it via mass transit, depending on what time of day / night you arrive and what your cost / benefit equation is.

There is a train from BWI to Union Station. I think you have to take a shuttle bus from the terminal to the train stop but it seems to be relatively close. IT is called MARC or MARK and it runs on the Amtrak line. Amtrak also stops there, but I think it is more expensive. I haven't done it myself, but when I take the train from DC-NY it passes through that station.

Its probably a 20-30 minute train ride to Union Station. From Union Station you can either take a $5-$10 cab ride or a $2 Metro (local train). My guess is the whole process is probably 1.5 hours when you factor in shuttle bus / train / changing to local train at Union Station, etc.

Taxi would be expensive (probably $50-$70?). If you are coming in late at night, it might be worth the extra money as it will be infinitely quicker. If it is late at night taxi should take about 40 minutes.

BWI might also have some kind of bus service to the metro station. The web site for the Metro is www.wmata.com. It has a "trip planner" where you can put in start at BWI and travel to "Metro Center" that is the most central train station in the middle of DC. Good proxy for wherever you will be going.

This is the BWI Amtrak station

http://www.amtrak.com/servlet/Conten...552552&ssid=94

MARK Train Info

http://www.mtamaryland.com/services/...mMaps/penn.cfm