To cost-effectively build a rail system in the US, you need to start with an FRA waiver (or avoid needing one with a fully separated system). Without one, you need special FRA-compliant trains that weigh (IIRC) 40% more than those designed to EU/Japanese rules and are of course at least that much more expensive.
Then you need to not take any federal funds, because if you do you'll be forced to buy everything from domestic manufacturers, which treat you exactly as you'd expect a captive audience to be treated. (This is why the US's shipbuilding industry is confined to government contracts and vessels that are below the minimum size to be covered by the Jones Act.) If you take federal money, also expect expensive and lengthy procurement grievances regardless of the merits. California probably has a similar state law, plus all sorts of others (e.g. CEQA), and the BANANAs are very experienced at wielding them.
Add all the union contracts that mandate excessive staffing, constant handing off of work between jurisdictions, etc. and you're paying ten times as much per kilometer vs. a comparable project in a high-wage EU country.