China is bigger and has a great high-speed rail network.

It’s less about size and more about population density.

Japan is 338 people per square kilometer en.wikipedia.org/wiki/Demographics_of_Japan

China is 147 people per square kilometer total en.wikipedia.org/wiki/Demographics_of_China

The US is 36 people per square kilometer www.census.gov/data/…/density-data-text.html

The US could build long distance high speed rail like Japan and China, but the ridership level would be rock bottom due to the low population density.

Demographics of Japan - Wikipedia

Those statistics are misleading. China’s population is very unevenly distributed between the east and west, but Western China is still serviced by high-speed railway.

The 1786km Lanzhou–Urumqi HSR serves three Western Chinese provinces: Gansu (57.7 people per sq km), Qinghai (8.2) and Xinjiang (15.8).