The closest MARC stop distance wise is probably West Baltimore or Halethorpe. Although they would likely require at least a 20 minute drive to get to the station. There is parking at both stations. The most convenient way to access MARC would probably be to jump on the Owings Mills Metro and ride it into Baltimore. You can get off at State Center and walk or take a bus to Penn Station (about 12-15 blocks away) to pick up the Penn line. The better way is to get off the Metro at Charles Center and walk about 5-7 blocks southeast and pick up the Camden line of the MARC at Camden Yards. Both the Penn line and Camden line go into DC and end at Union Station but the Penn line goes through East Anne Arundel and East PG county while the Camden line goes through West Arundel and Northwest PG county.
If you take the subway from Owings Mills into Charles Center Station, you can catch the 3 bus to Penn Station.