Actually there are none in Europe, but their density per square kilometer is something else.
Just kidding. However, that is why you look for density in specific sections of N.A. if you want to study the feasibility of HSR. The density in the Quebec Windsor corridor, as described above, is pretty close to the one in Spain. And in the USA, the density in the Boston-New York-Philadelphia-Baltimore-Washington compares quite favourably with European population densities. It's only the American love affair with their (humongous) cars and their penchant for super-multi-lanes highways, paired with the fact that it would be a multi-state undertaking that would involve so much politics (in view of the amount of money at stake) to adopt in the Senate and congress that its near impossible to achieve (can you imagine all the riders in the appropriation bill?)