air line
A railroad following a very flat, straight route between two endpoints in order to minimize travel times, choosing the shortest, most direct route practical even if longer route that was more advantageous was available.
air line: the straightest railroad between two points
An air line is a railroad route engineered to follow the shortest practical distance between two terminals, prioritizing directness over terrain or existing infrastructure. Rather than curve around obstacles or follow natural features, an air line cuts across country in nearly straight segments, much as a crow flies. The name reflects this principle: the route approximates a straight line through air rather than conforming to the landscape.
Railroad companies built air lines primarily to reduce running time and operating costs. Every mile saved meant faster schedules, lower fuel consumption, and quicker turnaround for freight and passengers. In the 19th and early 20th centuries, when competition for traffic was intense, time advantages were commercial advantages. An air line might reduce a journey by 20 or 30 miles compared to an older, winding route, cutting hours from transit time.
Building an air line required heavier engineering work than following a river valley or ridge. Engineers had to cut deep fills, build tall embankments, and construct bridges and culverts to maintain grade and alignment. The Pennsy's New York-Chicago air line and various western trunk lines were classic examples, with long tangent sections and gentle grades designed for sustained speed. These routes often incurred higher capital costs upfront but paid dividends over decades of operation.
The straight alignment also imposed constraints. Gentle grades became essential; steep hills that a winding route could climb gradually had to be avoided entirely or required extensive grading. Sharp curves were minimized. The result was higher capital expense but lower resistance to train movement and reduced brake wear. Some air lines incorporated short sections of steep grade compensated by easier stretches elsewhere, trading absolute directness for operational efficiency.
Air lines must be distinguished from branch lines, which served local traffic and intermediate stations, or from scenic routes that deliberately followed attractive terrain. An air line sacrificed local coverage and aesthetic routing for speed and economy. Once established, an air line route attracted long-distance through traffic and justified investment in modern signaling, heavy rail, and frequent maintenance. Abandonment of air lines in the diesel era, when speed advantages diminished and fuel costs changed the economics, left many of these engineered routes to decay or conversion to other uses.