We know for a fact there are numerous tunnels in DC, some that go to places to support / maintain continuity of government in the event of nuclear war.

I think it's erroneous to think that they are no longer in use, but nobody really knows their true extent.

The point was simply that if the amount of tunnels needed for this type of transport were dug, through the US, they would reveal, somehow, if such tunnels did really exist.