Baltimore has a beltway around it, which is called 695. It connects to 95 in two places, thenortheast corner and the southwest corner. If you have the misfortune of departing the airport or train station anywhere from 6:30-9:30 AM or 3-7 PM and want to traverse any of these main arteries, be prepared for gridlock and omnipresent, neverending road construction that is not lifted during rush hour. Try to schedule flights arriving during the middle of the day, if you can. 

The only option you have coming from BWI airport is to catch the Light Rail northbound only. The Light Rail does not intersect with the Metro or the MARC (none of them connect anywhere without having to walk a nice distance or hop a bus or cab), so tough luck and bring your sneakers if you want to go anywhere off the light rail line. To drive away from the airport, use either 295 North to 695, or 295 to 95 if you're going into Baltimore City itself. If you have to take a taxi, empty your bank account because it's not cheap...

You can get a quick shuttle bus to the BWI rail station, and from there take a train to Washington, Baltimore, or points north. And rush hour traffic in the Baltimore area is not nearly as bad as washington or other major cities: at the worst times allow half an hour from downtown Baltimore to BWI, and take I 95 during the current construction. Taxis are about $25 to $30 to downtown.