Amtrak and MARC trains already run from New Carrollton to BWI. MARC runs from Greenbelt to BWI. With the "National Agricultural Research Center", "Patuxent Reseach Center", and Fort Meade, there is almost no population between Greenbelt and BWI. Why in the world do we need a Metro line going through the woods?
Metro is a high density train so it would make sense to have an expansion stock in housing between Greenbelt and BWI, and if that area is connected then there will be a lot of development.