Sure can..its pretty easy. There is a stop labeled for Nstionals Park.
Yes, I regularly take metro to that area. From Vienna, take the train towards New Carrollton. Get off the train at L'enfant Plaza. Go upstairs to the upper level of the station on south bound Green and Yellow line side. Take the Green line towards Branch Ave. Get off two stops later at Navy Yard/Ballpark. Exit towards half street / ballpark. Exiting the station turn left. Walk approximately 2 two blocks. You are at the center field gate.
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