Went to Union Station today to checkout their Earth Day events. I have only been there once before years ago. It's been through a few renovations and boy is the architecture amazing. It is a train station served by Amtrak, MARC, and Virginia Railway Express plus the Washington Metro subway trains. Lets say it's one busy place. I will be back there's a gold mine of photos to be taken..got some fantastic news on my answering machine when I got home and I'll share that tomorrow on my year 2 album.
You are absolutely right... it's a city inside a city. I was there a few months ago to take a train to Baltimore and was really impressed. Yes, you should go back because there are many wonderful photo opportunities.