Natural monopoly/Talk

This was the original article. I'm not sure it jibes with my understanding of the subject, but maybe I misunderstand what's being said:

a specific case of a monopoly. It's existence is justified by the specific cost structure the monopolist, which barrs other competitors from entering he market by making it impossible for him to produce and sell his goods at a profit. A good practical example is a railway company - a competitor would have to build a second set of tracks in a specific area in order to be able to enter the market. Technically this implicates that the monopolists' long term average costs (LAC?) and long term marginal costs (LMC?) decrease when the output Q increases.

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LA2: Either way, you should keep the pointer back to monopoly. Pointers are always useful, whether they point to related or opposite words. (I have no informed oppinion on the economic theory here.)