The Baltimore-Washington International Airport has a very good website which includes a section about Ground Transportation. See it at: http://www.bwiairport.com/

Sara and I have transfered to D.C. by both taxi and train. Taxi is easier but more costly. We found the train to be very simple. A shuttle takes you from the airport to the closest train station (just a couple of minutes), the train takes you on into Union Station where you can catch the Metro or a taxi. The last time we used the train there was a major accident on the Baltimore Parkway with the result being that the train was actually faster than a cab.

John in San Diego