I think Union Station is a pretty impressive place. Yes, it is very mall-ified inside, but there are some splending spaces that have been preserved. And the exterior of the building gives you a sense of the thrill and prestige of rail travel in a bygone era. Great train stations were never merely utilitarian depots; arriving at Ronald Reagan Airport is just not the same.
Once you arrived at the Reagan airport, there is a train at the airport that will take you to downtown. It is a nice train ride through the town. If you lucky, you might see some of the high profile congressman riding on the same train.
I love the Metro and Union Station. The DC Metro is a model for what the rest of the US should aspire to.
This is a stop on trains to neighboring cities and a connection to subway. You can also buy souvenirs. Besides, this is nothing short than a monument of architecture.
If you travel via Amtrak to DC, you'll arrive at Union Staion.
Before you run out and grab a taxi, have a stroll around, it's a beautiful building on the inside and out.