MARC has provided a convenient way for commuters in the Baltimore and DC areas to reach BWI Airport. This station, which is also served by most Amtrak trains, is a short bus ride from the airport's main terminal. The bus ride is free, and there is a comfortable waiting room at the station. The buses operate approximately every 15 minutes, but be sure to arrive at the airport with plenty of time to spare - it will take longer to reach your flight due to the heightened security resulting from the recent increase in terrorism.
The BWI station offers a great opportunity for modelers and other people who want to take rooftop pictures of MARC and Amtrak equipment. There is a modern enclosed passenger overpass at the center of the platform, stretching across the tracks. After getting off a Washington-bound train (operating in push mode), you have just enough time to get to the top of the overpass to photograph the locomotive as it passes underneath. Hopefully I'll be able to add some photos of this soon.