You could always take the ICC 201 Commuter Bus from BWI to Gaithersburg (Shady Grove Metro Station)! It’s only $5 and it runs nights and weekends, more frequently than the MARC I believe.

I will honestly express my complete lack of sophistication when it comes to navigating public transit requiring more than one transfer but from what I’m assessing, you’d need to take the light rail to BWI ($1.60) then the commuter bus from BWI to the Shady Grove Metro Station ($5.00) then you can take the Metro into DC proper from there.

Thanks for the tip! This is a good idea for a way to get home after you’ve stayed at the bars later than the last MARC train (10PM on Saturdays & 7PM on Sundays).  Though I can’t imagine properly navigating this path home while drunk would be easy. The commuter bus runs til 12AM every day but the light rail stops running fairly early (12AM except 7PM on Sundays), so you may need to cab it back into the city from the airport which’ll run you $30+.