The OP is looking for suggestions for a 5 to 6 hour layover. Which is likely to be shorter because the odds are the LD train will get in late. With such a short window, really doesn't make much sense to visit paid attractions when there are so many free ones. Many within closer walking distance than the Newseum (which is about 0.9 miles from DC union Station walking distance according to Google Maps).I don't know how far away The Newseum is from Union Station, but it was one of my favorite places to visit. It's a paid admission museum, but it is worth a visit.
I would keep it simple - walk to Capitol Hill and Library of Congress (free) or pick a (free) museum to visit or sightsee a couple of monuments.