If you are coming from DC or BAlt, the MARC also stops at the BWI train sta. The Light Rail runs from Balt to the airport terminal.

Not all Amtrak trains stop at the BWI sta.

Trains starting North of NY, and South of DC to run the NY-DC corridor have a lousy on time record.

There is a shuttle from the train sta to the plane sta, but I do not know the schedule, or who runs the thing.