Washington is probably better than Baltimore.
If you're coming in on a North-South route through the Northeast Corridor, you should realize that the BWI Amtrak Station is more-or-less at the BWI Airport, and connected to the Airport via the Airport's internal inter-airport bus system. This means that someone arriving at BWI by Amtrak could conceivably hook up with someone flying into BWI. All that said, it's probably easier to get a ride with somebody form Washington, DC.

--Charlie Baum