The US has 90 cities with a population over 250k. Europe has three times that number. Population density in Germany is seven times what it is in the US.
Consider the cities of Paris and Hamburg (total population around 4 million) that stand less than 600 miles apart. In the gap between the two, you have Brussels, Amsterdam, Rotterdam, Cologne, Frankfurt, Stuttgart, Essen, Dortmund, Duesseldorf, Bremen, Hannover, and Duisburg. That's twelve cities with populations over a half million. There is nowhere in the US that comes close to that.
Europe also had the 'advantage' of not having to worry about private property rights after WWII. So they could build rail wherever they wanted for a population without automobiles.