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Air line

Posted by George Garrigues on October 12, 2007

In the 1880s or thereabouts, there arose a phrase, "air line," which seems to have referred to a railroad route running (I think) through open country in more or less a straight line between two or more cities, and I believe it referred only to passenger service. Why "air" line? Is the from "open air," or "air like bird can fly through"? Any solid source for this once very commonly used term?