That's a 20+ mile journey and they're thinking of putting the Metro line along the existing Amtrak right-of-way. Why not just use the Amtrak tracks? Kinda like Tokyo where the lines morph from suburban commuter rail into inner-city subways and vice-versa.

Also, that alignment puts the Metro a mile or so away from Ft Meade and BWI. The better alignment would be over 295 which abuts both locations. But I doubt the NPS will let WMATA do that.

Frankly, I would prefer that Metro concentrate on providing more service downtown than branching off into Dulles (NW), BWI (NE), Centreville (W), Ft Belvoir (SW), etc.