MARC Penn Line runs everyday. Take it to New Carrollton and ride Metro to the airport.

The Camden MARC line goes to the Greenbelt station. You could also park in Halethorpe, take the Penn MARC line to Union or New Carrollton, but then you'd need to transfer over to the yellow line at some point

Could take the MARC Camden Line and transfer to the metro at Greenbelt or College Park... probably not any faster, but depending on where you are in the city it might be more convenient then driving all the way to Greenbelt. Though not sure if the Camden line is running on weekends or not, so will depend on what days your trying to do the trip.

Penn is the only MARC line that runs on the weekend.

FYI the yellow line is currently being repaired. You need to take the Blue Line to get to DCA.

And look for signs simply to "National Airport" since nobody calls it anything but that. . .

MARC train to DC, Metro to Airport

There are two MARC train lines that go from Baltimore to DC: The Penn line is the main one, it goes from Penn Station to DC via New Carrollton, and runs every day with decent enough frequency.

The Camden line is the other option, it runs limit rush-hour-only service between Camden Yards and DC via Greenbelt/College Park. It is slower than the Penn line, but depending on your location in Baltimore and timing of your flight might be easier to get to.

For both lines, the fastest way to the airport is ride to Union Station, then take the Red Line to Metro Center, and change to the blue line for National Airport.

For the Penn line, if you don’t want to deal with changing at Metro center, you can also get off at New Carrollton and catch a blue line train directly to National airport, tho it will take longer than the Union Station option.

Normally if you took the Camden line you’d be able to catch a direct yellow line at Greenbelt or College Park, but the entire yellow line is currently closed for repairs to the bridge over the Potomac, so if you do the Camden line it’s easiest to just go all the way to Union.

Also, normally there wouldn’t be any blue line trains at New Carrollton, but due to the Yellow line construction, blue line service was significantly beefed up, with every other blue line train going to New Carrollton instead of Largo.

Lmk if you have any questions.

Über.

Take Amtrak/MARC from Penn Station to DC, then DC Metro to DCA.

MARC also goes from Camden Yards to DC but only during morning and evening rush. And MARC trains, especially newer ones, don’t really have a lot of space for your stuff.

When I flew out of DCA in May, I caught a Lyft from Union Station because I am a neurotic mess.