+1. I've arrived via Union Station late afternoon or early evening on a Sunday on more than one occasion in the recent past, and found myself waiting 20 minutes for a Red Line train, which was then further delayed en route.
On a happier note: another vote for the National Portrait Gallery. Among exhibits I've enjoyed there, I see that the show of Matthew Brady's Union Generals photos is still up:
http://www.npg.si.edu/exhibit/uniongenerals/
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