> Japan is geographically well-suited

Most of the rail has get around mountainous, uneven terrain subject to earthquakes, strong winds, and heavy rain. California should be able to build rail parallel to the I-5, a long, flat terrain without extreme weather or strong earthquakes. The problem seems to be a political one, not an engineering one. In fact, if the Interstate Highway System did not already exist, I doubt the U.S. today would be able to accept and complete it.

> one long line with a few branches

I currently live in Japan, and that does not really match what I've observed. There are three distinct railway companies in my area (JR, Tokyu, Yokohama Municipal Subway), each with their own dedicated rail, trains, power supply, etc.

The situation is more like "a disjoint union of graphs, where some of the graphs are connected".