OK, I realize this is your first post on Tripadvisor, but a little geograhy lesson is in order. :)
You posted this question in the College Park forum for some reaon. College Park is 20+ miles from Camden Yards, which is in Baltimore.
Arlington is in Virginia, and depending on which part of Arlington you end up in is about 35 miles from Camden Yards.
Metro and MARC don't work too well for an Orioles game. It "can" be done, but it's a long trip, and the MARC trains don't run after the game, you have to take a bus to a Metro stop or back to downtown Washington (Union Station).
Get a room in Baltimore, it would be much easier for you assuming you don't have a car.