Japan is almost all mountains, it's one of the worst geographies to build high speed rail where tunnels and turn radii need to be especially large. But they pulled it off anyway. The bullet train initially only connected metros like Tokyo and Osaka but today runs all the way to many remoter areas. The most recently added line connects Fukui prefecture, population 780k.

The US has many areas with suitable population density to be served by high speed rail, and with more accomodative geography than Japan. It's just that in the US, it was considered fine to use government funds and authority to bulldoze land for the interstate system, but not for high speed rail.