The SF Central subway project cost about $920M per mile, at those rates this $77B could be used for 84 miles of underground subway lines in the densest parts of SF and LA. That could be very substantial. (And not saying that's even the best way to improve transit, but it is probably the most expensive way and even still the money would go pretty far).
The US has 2 big problems:
1. “as a result of existing union agreements covering the eastern seaboard area of the United States, underground construction employs approximately four times the number of personnel as in similar jobs in Asia, Australia, or Europe.”
Holy crap, that means 75% of the personell are just sitting around not doing anything. Must be a very powerful union.
2. Regulation:
But the problems are not just to do with labor. Working alongside active commuter or intercity rail lines brings a slew of new requirements and regulations.
And the defense industry. The defense industry has the advantage of not only maximizing cost, but also maximizing states. For instance, I believe Alaska is the only state without an economic impact from the F-35, the most expensive fighter jet ever made.
SF Central subway cuts through one of the densest parts of SF (including crossing a BART line and an existing MUNI line), which substantially increases costs. Further away from the downtown core, costs per mile should be lower.
this is unfortunately true - however in this case it might be a municipal metro area body that comprises of one or more counties and multiple cities
the process of awarding projects to developers by government entities is very flawed and seems to guarantee substandard results and cost overruns.
edit: still having said this - it shouldn't be a reason to not try - we desperately need (and imo deserve) these improvements to our cities (as well as better municipal spending oversight as well as better planning)