It's the way to go to the ballgame. Take the Green Line to the Navy Yard station. The Navy Yard station is just a half a block away and will accommodate 24,000 passengers an hour. Plus, Metro has over 57,000 parking spaces at their Park and Ride lots.
Take a look at all the different Metrorail routes to Nationals Park
Metro is fully ADA accessible at the Navy Yard station. The Navy Yard West Entrance located on the SE corner of M & Half Streets, SE, as well as the Navy Yard East Entrance, located on the NW corner of M Street at New Jersey Avenue, SE, are both ADA accessible.
The Center Field gate is located a half a block south from the Navy Yard station on Half Street.