What's the cheapest & fastest way to get from Reagan Int'l Airport in DC to Baltimore In'tl (BWI)?

Yellow Line to Gallery Place
Red Line to Union Station

MARC Penn Line to BWI Airport (weekdays only) - take shuttle bus to terminals

Optional - Stay on Yellow Line to Ft. Totten & transfer to Green Line to Greenbelt to B-30 Bus (daily)

Take the yellow line to the convention center then take the green line to Greenbelt, take the B30 express bus to BWI, if these are working at the time you get there. After all the Metro does close down at times.

This is my help from Washington, D.C. United States of America.

drive.

get the express train from Grand Central Station

First of all, it's Reagan (snarl) National, not international. I will forever curse the Congressidiots who insisted that man's name be plastered on that airport and the attached Metro station. Take the Yellow Line from National Airport towards Mount Vernon Square (during nonpeak hours, it goes to Fort Totten) and transfer to the Green Line towards Greenbelt. At Greenbelt, take the B30 bus, which runs every 40 minutes, to BWI. It's an express bus.

Don't forget to get a transfer before you get on the train at the airport station. You'll get a discount on your bus fare.