There's also Amtrak, probably about 12 bucks or so... Last time I was up in BWI
(maybe 3 weeks ago?) The mass transit system around the airport was undergoing construction so it made it a little more difficult to travel. I wanted to take the Light Rail into downtown, but had to take a bus from BWI
(took about an hour for the bus to show up!) then had to hop onto a train downtown.
I've never used the MARC, but I do know people that use Amtrak on this service and have no problems.