To do so, of course, he'll need to reduce the costs of tunnel boring by an order of magnitude. As the economist quoted in the article noted, though, the construction industry has been famously stagnant for decades. Commoditizing boring machines would bring down costs substantially. Speeding them up would also reduce costs. And if he gets costs down by a factor of two or more, he could be the preferred vendor for a truly massive market worldwide.
It might be his least crazy idea, honestly.