Air-line railroad

An air-line railroad was a railroad that was relatively flat and straight, choosing a shorter route over an easier route. In their heyday, which was prior to aviation, they were often referred to simply as "air lines". For example, a 1903 novel indicates a character's success by noting his position as "superintendent of passenger traffic of the New York and Chicago Air Line", a fictitious railroad.
Dictionary definition.
Webster's 1913 dictionary gives the definition "Air line, a straight line; a bee line. Hence Air-line, adj.; as, air-line road." ( this meaning has fallen into disuse.) 
Satire.
Air line railroads began to be built in the mid-nineteenth century; in 1853 the "New York Daily Times" ran a satirical article mocking the trend, suggesting that the fad for an "air line" name was being used to float dubious investments: