Von, car service or limo will cost an arm and a leg. Can you rent a car? Ashburn is about 35 - 40 miles from the train station. If you can get a ride from the Tyson's Metro station, it will make the trip shorter. Tysons is right on Route 7 which is a straight, albeit very congested, shot to Ashburn. You could take the metro from union station to there. MAybe you could get Enterprise Rentacar to pick you up at the metro in Tysosns and then you could go from there. Then return the car to the Enterprise office and have them take you back to the Metro station.