Earlier quoted context omitted.
"Economies of scale" when tunneling through solid rock?
I would naively expect running a tunnel boring machine 1 mile to cost a lot more per mile than running it 2 miles.
FWIW, a surprising amount of the base tunnel was blasted by dynamite, as the geology didn't allow use of TBMs everywhere. (And they hit more cases where they had to blast it than they expected.)