I wouldn’t recommend the metro. It sounds easy but it takes a while, you’ll need to switch lines and if you’re not familiar with the city it will probably just stress you out needlessly. I would try to get a taxi from IAD (cheaper than Uber! But expect to pay about 60 bucks) and go right to union station. If it’s too late in the day, book a room at the Phoenix Park hotel (across the street from union station) stay the night and grab a train the next day. I throw money at things that stress me out so I guess take this advice with a grain of salt if you’re trying to keep this trip lower cost.