Sure, the TGV is a standard-gauge set that can run in most of Western Europe's rail.
The issue is that regular rail can't run on the TGV lines.
Japan/France took a different approach than Germany/Switzerland for HSR. Germany and Switzerland are far less centralized than Japan and France, and that reflects in the network, which began as upgrades to lines between major cities, and new sections with new tracks.
France's system reflects its historical centralization, since the late middle ages. Paris is the economic, political, artistic, demographic center of the country, and just like the first railroads in the country were created to expand central control over the rest of the country, TGV started as a spoke network with a single hub: Paris.
The issue is that regular rail can't run on the TGV lines.
Japan/France took a different approach than Germany/Switzerland for HSR. Germany and Switzerland are far less centralized than Japan and France, and that reflects in the network, which began as upgrades to lines between major cities, and new sections with new tracks.
France's system reflects its historical centralization, since the late middle ages. Paris is the economic, political, artistic, demographic center of the country, and just like the first railroads in the country were created to expand central control over the rest of the country, TGV started as a spoke network with a single hub: Paris.