Seems there's no train to take you into the city centre..
You're right - no train... not yet at least. WMATA (local transit authority) are trying to get the Metro out to Dulles but it's taken a while for them to get planning and funding in place. The work has finally started but it's going to be a few years till it's linked up!
In the meantime yes, Washington Flyer or taxi or limo!