What's interesting is dozens of message on HackerNews and nobody links to any type of studies or evaluation of Elon's work, just repeating is selling point that because the tunnel is smaller, it will be cheaper. Public transportation and cost is a very well studied topic internationally (outside the US) and we can't just assume that for a fact. Actually, boring is actually not the most expensive part of building a subway. Even this idea of saving 10x the amount of money, actually just works when you compare with other US subway construction projects. Close to 10x saving has already been achieved in Spain and in South Korea, and surprise, it didn't need some new fancy technology.
Great article here: https://pedestrianobservations.com/2017/12/15/elon-musks-ide... Basically