> anywhere but places with the most dense rail networks per sq km
It seems to be the contrary to me. In Japan or France (in the denser places of these countries, because they have plenty of low density areas) building a whole new dedicated rail network was (and still is, see the Bordeaux-Toulouse LGV project) a major undertaking because there's no way to avoid built-up areas. The high-speed lines go from city center to city center, and we're talking about Paris and Tokyo here.
The Shinkansen and TGV use dedicated rail networks, that are built from scratch and are still being expanded. TGV can use legacy lines as well at low speed (including at grade road crossings, although these have almost completely been replaced today) not sure about Shinkansen.
In contrast, it seems to me like it should be easier in the US to find space for cheaper bypasses, tunnels and bridges since it's less dense.
It seems to be the contrary to me. In Japan or France (in the denser places of these countries, because they have plenty of low density areas) building a whole new dedicated rail network was (and still is, see the Bordeaux-Toulouse LGV project) a major undertaking because there's no way to avoid built-up areas. The high-speed lines go from city center to city center, and we're talking about Paris and Tokyo here.
The Shinkansen and TGV use dedicated rail networks, that are built from scratch and are still being expanded. TGV can use legacy lines as well at low speed (including at grade road crossings, although these have almost completely been replaced today) not sure about Shinkansen.
In contrast, it seems to me like it should be easier in the US to find space for cheaper bypasses, tunnels and bridges since it's less dense.