Before the widespread advent of condominiums, one of the most prevalent application of air rights involved railroads. With the density of many metropolitan areas and the advent of stronger construction materials (particularly steel and concrete), many railroad companies made extra income by selling or leasing their air rights.
The most common, but overlooked, example of using air space above railroads is street overpasses above railroad tracks. When the local, state or federal government builds an overpass above train tracks, they first had to obtain the air rights over those tracks.
A more discreet application has become more widespread in 20th-century cities, where land is so valuable. The Merchandise Mart and Central Post Office in Chicago, portions of the St. Louis downtown riverfront and the Park Avenue developments in New York are all examples of developments over railroad tracks. In each case, the railroads sold or leased the air rights above their tracks to allow developers to build above them.
Of course, some of ground and sub-surface portions were also transferred so that columns and caissons could be built to support the developments above. Caissons are typically from the ground surface down to several feet below the surface; the columns would then rest atop the caissons.
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