A bit confusing. Are you planning on connecting to a PHL-BWI flight at PHL and then to a train in Baltimore? If that's the case, isn't the decision based on the amount of time required to travel from BWI to the train station and the train departure time? I mean you obviously don't want to chance missing the train. As a backup, it's an almost seamless trip, via the in-terminal train, from PHL to the Philadelphia AMTRAK station (a 20 minute trip) and thence to less than hourly trains to Baltimore - a 1-1.5 hour trip.