How do you get from Washington D.C. to Baltimore Washington International airport via public transportation?

From DC you need to make your way to Union Station. There are plenty of metro trains or buses that can get you there.

At Union Station you can either take a MARC train (less expensive) or Amtrak to BWI Rail Station. Amtrak has a wider service schedule (weekends and early and late trains)

Marc trains provide service Monday to Friday to Baltimore Penn Station - $4 one way - and Union Station in Washington DC - $6 one way.

Amtrak has daily train service to Baltimore Penn Station - $12 and up one way - and Union Station in Washington DC - $12 and up one way.

From BWI Rail station there are free shuttle buses that go to all terminals.

Enjoy you trip!