I've written elsewhere about Union Station's fast food court, among the best places in the city to get a quick and delicious bite. Let's look now at the rest of the building, the train station/shopping complex/National Visitor's Center that is Union Station.
Union Station is THE Train station in Washington, DC. Metro stops here, of course, and Greyhound is only a few blocks away, but it is the Amtrak Station, which means that travel up and down the Eastern Seaboard happens from this spot, and two different commuter train lines, VRE from Virginia and MARC from Maryland, also stop here.
The train station has all the amenities you'd expect of a train station, and reminds me of European train stations. Near the tracks are the services, rental cars, currency exchange, newsstands, travel shops, ice cream and pretzels, shoe shines, and ATMS. Further out you'll find a two story mall with unique and often expensive wares. Further out from that you find a grand entranceway, a room full of unique gift and art sales, fine restaurants, and the grandeur of the train station in a capitol of the world.
Even if you don't arrive in Washington via Union Station, consider coming here just to drink in the ambiance, have a good meal, and rest your feet. Dining here ranges from the fast food in the basement (see my dining entry) to linen tablecloths.
Union Station is at the Union Station stop on Metro's red line. It is three blocks south (on 1st st) from the bus station, and on MARC, VRE, and Amtrak lines.