Amtrak has (or had) a deal with MARC where they let MARC monthly passholders ride a few select northeast Regionals between Washington and Aberdeen. When I started riding, they had some deal where you could buy "step-up" tickets between Baltimore, BWI, and Washington. I would have liked it if they had a deal to allow MARC passholders to ride the Northeast Regionals between Washington, BWI, and Baltimore. It would have allowed MARC to open up more seats on the local trains to people going to local stations, and Amtrak, even if all they got was the much cheaper MARC fare, would have filled empty seats between Washington and Baltimore. And, believe me, the local trains would have benefited from getting the Baltimore passengers on to Amtrak -- there was a while when the MARC locals were standing room only.