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 or the Orange, Blue or Silver Line to the Capitol South station, just a 15 minute walk from the Ballpark. 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
Fans attending games at Nationals Park are encouraged to visit Metro's website at wmata.com for up-to-date information.