Antitrust isn't solely limited to natural monopolies. (Also most of the time, what you describe wasn't quite the case—there's many places were there's two, usel

Antitrust isn't solely limited to natural monopolies. (Also most of the time, what you describe wasn't quite the case—there's many places were there's two, useless railways running parallel for each other for stretch, not for any technically useful reason—just an economic cooperation failure. You can call the monopolist's bluff and build a redundant (n+1)th rail line or an (n+1)th orbital launch company—it's just devastatingly expensive).