Why?

If I'm willing to take the train slash underground system from (DCA) Reagan International, why wasn't my brain equally willing to take the MARC train from BWI (Thurgood Marshall/Baltimore-Washington International) in order to land back into the City?

Obviously I fail at geography once more. I think I will opt for Delta as ATL/BWI with checked luggage which would still be less than Delta ATL/DCA with no checked luggage. Else, I should leave my Leatherman kits at home and buy another one while I'm in NYC (again!)