I can’t believe California pays 1 billion dollars a mile just to drill out tunnels. I think the going rate in Europe (not even China!) is something like $40M/km or fifteen times less than what California is paying per mile.
Even in one of the largest cities in the country? Can you dig a tunnel in London, Berlin, or Paris for that price?
Berlin calculated some planned subway extensions (U1, U8 lines) at $120M to $280M per mile (2017), tho, those are short extensions of existing tunnels of about 1 km each and only estimations not actual costs.