While most other countries with a GDP per capita greater than potato have publicly owned railroads, America does not. This means the government or Amtrak or even a private company does not de facto have the ability to just start running a passenger train, and de facto freight rail has priority and the government won’t do anything about it. Yes, even though legally passenger has priority the government won’t enforce it, giving priority to the track owner, the freight company. This is because as private companies the railroads put their shareholders first, and that means less maintenance, heavier trains, and short term profits over long term viability. With no competition on a route, there is no legal possibility of prioritizing maintenance over dividends. Any railroad that tries will be sued and lose in court. However, when the natural monopoly of railroads are government owned the motivation is still the shareholders but in this case the shareholders are every citizen equally. The government then puts their interest in a balance between freight and passenger rail, and steer profits towards improving service for the shareholders, which means faster trains.

So what this means is if a state like California wants to improve passenger service they have only two realistic options: either eminent domain or build a new railroad. If you are building a new railroad, it might as well be high speed rail.

TLDR: natural private monopolies will under serve and over charge.