Indeed, that's really only true because the United States and Canada have vast areas in which nobody lives. About 1/3 of the U.S. is federal land managed by the Bureau of Land Management; that really throws off the average. If I recall correctly, around 40% of the population of Canada lives along a line between Windsor and Montréal. Meanwhile, this is Spain compared to the Northeast:

Spain has a population just under 50 million. It also has a high-speed rail network. You'd think that in a region with a far, far higher population density, the U.S. and Canada could do better than the Acela.