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Europe also has ~3x the population density of the US, higher still if you don’t include some of the sparsely populated Eastern European States.

While I can’t profess any expertise on this topic, it strikes me as a possibly reasonable hypothesis that the sheer emptiness of much of the central US could have some kind of bearing on trying to make sustainable continent-wide public transport more difficult than in Europe.

Colorado for example, roughly the same size as the entire U.K., has around 50 people per square km. The U.K. has over 300. Even France, regarded in Europe as a largely rural State beyond a few large cities, is over 100.