$77B was after they did severe "cost cutting". It used to be $100B. And, if the past is any indicator, it would have ballooned to $120B - $150B by the time it was done. At what point do you say it's not worth it?
Personally I think it’s still worth it at that price, even if the project would’ve been better done in the way I described. It’s better than the alternative. If we don’t spend $77b on rail we’ll still have to spend it expanding freeways and airports instead. and infrastructure costs are insane across all forms of infrastructure in the US.
How much would the train need to charge each passenger to earn enough money to pay both the interest (assuming normal cost-of-money) and the principle? Would this amount be less than the cost of an airplane ride?
The alternative is spending money on rail inside SF and LA. Connecting the cities is a solution to a non-problem.