Originally Posted by
paytonc
The same article mentions that the percentage of US arrivals who are connecting has gone from 18% to 40% -- presumably, those are additional passengers from their expanded operations.
Tunneling and excavation are never cheap, but get much more expensive when done in areas below the water table -- like the sea-level muck beneath DCA. You need to continuously keep water out of the hole, which requires pumping 24/7 and lots of additional concrete.
This is likely the answer. I'm told that tunnelling is essentially not viable given the disruption it would cause for lengthy periods of time to existing operations. I do wonder whether there's any chance of turning what was the temporary US/DL terminal (old hangar, now offices) into a temporary terminal again if the tunnel option were ever to come to pass.