The trick is the Amtrak/MARC service from BWI to Union Station. The station at BWI is a ten minute coach (not van) ride from the terminal. Weekend fair requires an Amtrak ticket which is 26$. On weekdays, the MARC service is running and the fair is $6. The trip is about 35 minutes. Union Station in DC is on the Red Line Metro and you can get just about anywhere in DC with a short metro ride.