Define "Europe." "Europe" doesn't have high speed rail. Germany, France, etc., do, and those countries are a lot denser than the U.S. and most U.S. states. Moreover, U.S. cities are shaped very differently from Western European ones. Take the ratio of (city population) / (metro area population) and compare the U.S. state capitals to European capitals. In Europe, it's common for the majority of a metro area to live within city limits, even in small cities. In the U.S., the only major city that gets close to that benchmark is New York.