The Westin Crown Center in Kansas City is perhaps 100 yards ..... maybe a shade more.... via enclosed walkway from Union Station. It's also attached to a small shopping mall so it's possible to go from train to hotel room to some eating options, shops, movie theatre, etc. without facing the outside elements. Valuable if it's raining, snowing, or excruciatingly hot or cold..... all of which are possible in Kansas City.
I've stayed there twice on train trips though on both trips I've gotten the room when it was offered as a special. The regular Westin rates are a bit out of my normal price range.