"Straight as a plumb line": Seaboard Air Line Railway advertisement illustrating the "quickest train service via the shortest route" to Florida, 1902."The straight line of your palm is not straighter than the straight line to the palms": another advertisement for the Seaboard Air Line's "shortest, quickest route to Florida", 1908.
An air-line railroad was a railroad that was relatively straight, following a shorter (and thus, presumably quicker) route instead of a longer, winding route. In their heyday, which was prior to aviation, they were often referred to simply as "air lines".
