I am no expert in the slightest (I would check the things I linked), I'll try to answer 1) Mountains actually helping is my own pet theory, I should disclaim.

I am no expert in the slightest (I would check the things I linked), I'll try to answer 1) Mountains actually helping is my own pet theory, I should disclaim. Japan does seem to have wild swings in density, while England seems to have people/towns strewn about everywhere and London feels kinda sprawly at certain scales. That said UK obviously has cities / regional clustering at the scale of HSR. Someone should do a proper (fourier?) analysis of sprawl at different different scales and how that effects rail at different scales and until then my theory is just idle speculation, and in any event I don't think Britain can or should blame it's geography when other flat parts of Europe do better, and the Anglosphere clearly sucks at rail on the whole. 2) So Europe is kinda of anti-modern avoiding dense downtowns / CBDs, America is downright evil with it's single family homes, but weirdly still has tall towers people curse driving too. East Asia is largely "correct" in having dense workplaces and living places, which you want if your goal is both less car trips and classic max GDP growth. I don't think the UK should be like Japan in al respects, but London is emphatically way to evenly low-rise, housing is not cheap enough I hear, and all those rich people LARPing landed gentry commuting 3 hours from their quaint South England rural hamlet is a sure sign something weird's afoot. If they get rid of the green belt because no one wants to build taller, I will be sad. 3) https://en.wikipedia.org/wiki/Japan_Railways_Group https://en.wikipedia.org/wiki/Japan_Railways_Group the story is maybe more muddled than I thought, or maybe that's just the terminology for historical reasons. 4) Well, that I think goes back to the unique economics of rail. I think good rail becomes profitable, but profitable bad rail doesn't become good or isn't even possible!