Having the train station downtown is great if that's where you are going. A lot of traffic in and out of BWI is actually for the DC area, and of course there is an Amtrak station which serves BWI with a shuttlebus.

The passengers that I see riding Amtrak out of Pittsburgh are retirees, college kids, the Amish, bicyclists, train buffs, and assorted weirdos. I don't see business people, because under the most delay free conditions the trip takes 8 hours to both DC and NYC. By airliner it's an hour and a half. It's even faster by car or Megabus, at around six hours. So Amtrak will be the slowest option no matter what.