If you will be relying primarily on public transportation during your visit, Washington National will be your best choice, as it contains a Metro stop.
If you plan to rent a car at the airport, Dulles and Washington National are probably more convenient options than BWI.
For those arriving by rail, Amtrak trains arrive at Union Station in Washington, DC. From there you can either take a taxi or the Metro to suburban Maryland and the seminar hotel.
|Metro Directions: Washington National to Silver Spring station (seminar hotel).|
|Metro Directions: Union Station to Silver Spring station (seminar hotel).|
|Washington Metro Map|