To be fair, light rail is cheaper to build than high-speed rail. And if we are going to fantasy-land where the government can do things for the public good without caring about who it inconveniences right now, they could build light rail on land "the government" already owns: in cities dedicate lanes to public transit (as in light rail + bus), to go between cities take some highway space. Rail has much higher capacity than cars, so if you get public transit right this should actually decrease traffic after it's built.

Of course this makes it obvious why California can't do it for $10 billion, $100 billion or even $10 trillion: wherever you put your rail network it will inconvenience someone, and that causes massive delays and cost overruns, or makes the project politically untenable from the start. Even if you put the rail underground you would get massive protests from nearby residents because "poor people" can now get there easier.