There's one that runs from BWI directly into Union Station in DC, which last time around was where the DUers met.
http://www.bwiairport.com/4grndtrn/in_mrct.shtmlOn weekends, though, it might just be regular Amtrak service.
Here's some other info:
You have plenty of transit options here. Maryland Mass Transit Administration (http://mtamaryland.com) provides subway, light rail, and rapid bus service to Downtown and major employment centers. The subway connects Johns Hopkins Medical Center, Downtown, Mondawmin Mall, Northwest Baltimore and Owings Mills. One light rail line connects Hunt Valley, North Baltimore, Downtown, South Baltimore and Glen Burnie. The other light rail line connects Penn Station, Downtown and BWI airport. The airport station is walking distance to the terminals. One MARC commuter rail lines extends from Camden Yards downtown to Dorsey, Laurel, College Park (U. of Maryland) and Union Station in Washington, DC The other MARC commuter rail line begins in Perryville (northeast), Martin State Airport, Penn Station, West Baltimore, BWI Airport, Bowie State University, New Carrollton and Union Station in DC. The MARC/Amtrak/BWI station bus shuttle bus shuttle runs every 10-15 minutes and parking at the station has been greatly expanded. In 5 years a monorail will directly connect the MARC/Amtrak/BWI station to airport terminals. MARC fares between Baltimore and DC cost only $5.50, but the train has lots of stops. If you are in a hurry catch Amtrak.Good luck, and I hope to see you on the 25th!