I got the train to Washington DC from New York so had to set out very early but it was worth it.
If you don't have long in Washington DC, I'd say that the best thing is to get tickets for an open top bus, you will see so much more as most of the attractions are really spaced out. The stops are near the main big attractions and you can get on and off as much as you want.
My first stop was the Capitol Building which only took a few minutes on the bus. It is a fantastic big building and if you speak to any of the staff, they will tell you all about it. It easily takes about half an hour to walk around the outside of the building if you want to see it. There are tours but the day I was there, the tours were cancelled due to work that was being done.
The Washington Monument was next but on the way on the bus, I got to see some of the Smithsonian buildings and The Wharf before getting off at my stop. You can go into the Washington Monument but it was really crowded when we went so I decided to give it a miss.
Next stop was the Lincoln Memorial so I got to see the Thomas Jefferson Memorial on the way. Oh wow, the Lincoln Memorial was just so wow. You know when you see something in films, books and magazines and then when you are actually there in front of it, it just leaves you speechless. That was it for me. I will admit, I did think it was going to be bigger for some reason but it was still magnificent. I walked all the way around it as well as and took in the great surrounding views.
I did want to go to the White House next but it had been put into lockdown due to an overseas threat so my bus got diverted. I stayed on the bus and got to see the old FBI Building, Natural History Museum, the Spy Museum, the White House (from afar haha), other memorials and loads more.
Union Station isn't just a train station, it has lots of shops and restaurants inside as well so if you are early for your train, you can go to get something to eat.