I visited DC last May and noticed all the construction...everywhere...but didn't pay much mind to it, it is a big city after all and cities tend to have construction projects going on all the time. Well we returned, and it was still happening, that's when I was curious. Turns out that right outside of DC, in the suburbs, and mainly focusing on Tysons Corner, they are extending the DC subway system to Dulles Airport.
You can see more about the project here, with pictures and an interactive map.
Has anyone been by this project?