Since no one else has noted it: they show rail lines and only rail. No roads on those maps. This includes some quite obscure ones like the railway between Labrador and Sept-Iles, Quebec. (It has almost no traffic and it serves a small town and a mine and it's not connected to the rest of the North American system.) Similarly they depict sections of rail in Canada that were out of service many years before this map was published. So they're quite out of date. To not show that Canada is linked by rail with the USA at Detroit is a definite oversight, too.
Seeing through the lens of railroads is probably an artifact of both ideology and the economic reality in North Korea. And maybe also the implicitly military purpose of these maps.
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