Or we still haven’t, depending on how you decide which is the biggest airport. BWI had slightly more passengers than Dulles last year; Dulles had slightly more flights than BWI.
It has light rail service to Baltimore, but the Amtrak/MARC connection to DC is many miles away from the airport, connected via an awful shuttle bus that dawdles around the woods wasting 20 minutes of your day. Plus the trains are extremely infrequent. The option exists, but only in the most nominal way.