Switzerland is about as big as Vermont and New Hampshire.

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.

Could be, feel free to post a similar study on that region if you find one!