I just flew out of IAD this weekend and since I had some time to kill, I figured I'd take the new underground walkway from Main to Terminal A (just to see what's there) and then take the shuttle bus thingies to Terminal C. Once in A, most signs said to get to C, I'd need to go back to Main. One sign, though, said to go toward the A/B borderline and get an A to C bus there.

So I walked down to A/B border... and found nothing but a side passageway blocked off by drywall. Not sure if I walked into B if buses maybe picked up from there, but I gave up.

I walked back to Main, and took shuttle to C.

YMMV,
Alumino