Is there a shuttle or train that do sightseeing for people who have long layover at Dulles? Is it advisable to leave the airport for a few hrs on a layover flight?

There's nothing much to see and the traffic gets really bad

Better stay at airport nothing special nearby to see. Minimum one and half hour drive to DC of you wish to see White house and Lincoln's Memorial

Relax and stay in the air port because need. About 50 minutes to cross the security sistem

Yes there is the shuttle bus and the train to sightseeing at the airport to watch the planes. On the other hand don't be late for your flight , okay . Don't leave the airport .😀🇺🇸✈