Originally Posted by
ColumbiaPete
The MARC train that stops at Greenbelt is on the Camden line and doesn't go to BWI at all. You would need to get to Union Station in Washington and catch the Penn line MARC to get to BWI.
madecon is referring to the B30 express MetroBus that goes from Greenbelt Metro to BWI. It's a great option during non-peak hours, but I would take MARC from Union Station since you don't have to deal with traffic issues.