Originally Posted by
Wiirachay
Get off at Alexandria, VA, one stop prior to DC. However, make sure you called a taxi in advance. Try Red Top Cab or White Top Cab.
Bear in mind that a taxi from Alexandria to IAD will cost about $60 (including tip).
You could, however, still save time by getting off at Alexandria, walking across to the metro (King Street station is pretty much adjacent to the Amtrack station), and then using either of the more usual metro + bus public transit options. Yellow Line will take you to L'enfant Plaza for the 5A, Blue Line will take you to Rosslyn where you can either get the 5A or transfer to Orange Line to West Falls Church for the Washington Flyer bus.