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One third of the population of Switzerland lives within 5 kilometers of the main train line crossing the country
(reddthat.com)
Source (yes sorry it's LinkedIn, if you any another one, I'll update): https://www.linkedin.com/posts/prof-stefan-michel_notsweden-train-activity-7210174690827370496-8iJz?utm_source=share&utm_medium=member_ios
Not as big a portion of the country, but yeah coastal areas will often have a large population living in roughly a line.
In North America I believe the line connecting the most people would go from Quebec City through Montreal, Ottawa, Toronto, London, Detroit, Chicago, Milwaukee and Minneapolis (though maybe not within just 5 km of it). This is the most populous part of the Great Lakes Megalopolis and into the St. Lawrence.