Union Station / the Train, Washington D.C.
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.
I love the Metro and Union Station. The DC Metro is a model for what the rest of the US should aspire to.
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.