I disagree with your assessment because American density is very different than European density. If you look at Google Earth, there is almost continuous suburban development along the entire Northeast route. In Europe, populations are clustered in smaller urban centers. High speed rail can only stop so many times and because of this, it is more difficult for the average American to get to a train station.

This situation is obvious to people who have lived in suburban America but can be surprising to those who have not. Imagine driving up to an hour to a city center, finding a place to long term park your car near the station and then taking a mode of transportation that isn't significantly faster than your car would've been. Then needing to take an expensive taxi once you arrive because your destination city has a limited public transportation system.

Comparing region densities at face value misses the issue, the way cities and regions are designed present unique transit challenges around the world.