A couple of helpful hints for taking the MARC:

1) If you're hauling luggage, try to get to the train early (say, 15-20 minutes ahead of time). Most of the afternoon rush MARC trains on that line use double-decker cars with woefully little storage space for anything larger than, say, a briefcase. Not to mention they're often standing-room-only, packed with commuters. Storage space for large items can be found at either end of the double-decker cars. Don't plan on stacking your luggage in the seat next to you.

The single level cars will occasionally pop up on this line. The overhead racks above the seats on these cars have plenty of room for luggage.

2) (And this goes for AMTRAK too) The northbound platform at the BWI station is short and not all the cars will make the platform. So make sure you ask the conductor on the platform to confirm which cars will make the platform. Generally, on AMTRAK, it's the cars toward the front, and on MARC, the cars toward the middle and rear.

3) The bus to the terminal stops on the road between the two garages. A mass of people will assemble at the stop at the close end, but there is a little-known second stop near the other end of the garage. If you go the taxi route, $5 seems to be the going rate.