The French already said they could build the California high speed train for half the current estimate, or something like that. As mentioned in other comments,

The French already said they could build the California high speed train for half the current estimate, or something like that. As mentioned in other comments, it's the popular opinion and government fragmentation in the US that makes this impossible. In France, the train builders and operators are nationalized (roughly) and the central government is much more powerful (not in terms of absolute power but rather in public acceptance). So in France, where high speed trains have been successful for years, the state run constructors can get eminent domain, lay track and run trains with only a few nimbys that most people ignore. Another way to see it is if Parisians want to take a high speed train through your back yard to get to the ski or sun, too bad for you. That doesn't happen in the US with weaker central government and stronger property rights (for corporations too). Japan and China are the same way (centralized command-based government).