Soooooo Amtrak or your commuter rail?

From DCA  metrorail or cab to union station and then Amtrak or Marc train to BWI airport.  You could also use uber.

Just take the metro to union station redline then amtrak to BWI it's very simple...

You can get MARC or Amtrak from various metro stations, you can get the train up to Greenbelt and then a bus to BWI, or get the subway/metrorail to Union Station and get Greyhound or Megabus (you can also get MARC/Amtrak from there) or roll Uber/Lyft all the way if you're feeling baller.

OK sounds moderately confusing but I am tired from walking around Boston all day long. I will write everything down and then google it all. Thank you all for your help. I appreciate it very much. xo Erin