MARC trains, which only run on weekdays, cost about $7 from Union Station to the BWI
station. AMTRAK, which runs at least once an hour, will cost about $13. From Dulles, there's also the 5A
Metrobus for $3 that will take you to either Rosslyn Metro Station or L'Enfant Plaza Metro, and from either two you can get to Union Station, though you will have to transfer within metro to the Red Line at Metro Center. The Washington Flyer bus runs every half hour, and the Metrobus runs every hour. I hope this is helpful.