Trade Routes Explained... please?

I can't say I understand how the game determines what trade routes are possible in any given city. I've had costal cities that would only trade with my capital and core for +2 and landlocked cities that would trade with faraway lands for +9 or more, with open boarders the whole time. Is it a function of distance? Is there a threshold on how far your cities should be from your rival's core?
 
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