japan is a small island the US is one of the most extensive and biggest distance from population centers country on earth
I tihnk that helps explain the feasiability of train on each country more than inherent choices
Civil planning on that scale isn’t about feasibility but about what direction you want to shape the county in.
A sparse railway system would leave parts of the country less populated by design as it’s simply harder to get to them. People would bunch up into cities and towns because they had to.
The US has several areas of high population density that have laughably bad rail networks.
It’s not just about size. Much of the U.S. would be cheaper to build rail networks because there is a lot of open, relatively flat land without dense building on it. Japan is very mountainous and has a lot of dense development, and it has to be more resilient in case of earthquakes.