You can drive to Halethorpe in maybe 20 minutes from Hampden across the city, and Halethorpe usually has free parking depending on when you arrive. At that point you're already nearly at BWI, slightly shorter MARC ride. It's kind of dumb with traffic, but certainly doable. You can also certainly ride your bike from Hampden to Penn Station, and from there you can even take the Amtrak for a faster but slightly more expensive ride than MARC.