Only a crazy person would EVER recommend IAD over DCA when going to Baltimore. As a DC resident, always go with DCA! And a regular Amtrak train (Acela Regional Service) (but NOT the Acela Express) from DC to Baltimore runs $18, NOT $50. The MARC train is definitely cheaper and definitely slower as well. It is also much grungier than Amtrak. I live here and I know for sure. For the extra few bucks, I take Amtrak and also get AGR points as well.