Recent travels have taken me to Chicago, San Francisco and Paris (and explain the lack of activity on this page). It is the latter of these cities which I am going to use to explore the above-city landscape.
What is the above-city landscape? Well, in the late 1800s and through much of the 1900s, cities realized that it was generally quite easy to build transportation networks above street level. The first of these took the form of steam-powered, elevated railroads. In most cases, these were built on metal structures above the street, but in a few, they were built as stone or masonry structures instead. The next generation were electrically-powered elevated railroads, which were mostly built in the early 1900s and, in many cases, torn down during the latter half of the 20th century, which were followed by, after 1950, mostly concrete elevated road structures.
Most of these structures, especially the narrower, non-road ones, were built over existing roads. (Road structures are often several lanes wide and required significant property takings, as there were no existing rights of way wide enough to carry them.) Thus, when they fell in to disuse or when they were made redundant by paralleling surface or underground routes, most were seen as a blight to the landscape and torn down. Metal, over-street elevateds are easy targets: they are ugly, they block light, they generally carry noisy traffic and their supports impede the flow of traffic. If they no longer serve a purpose (such as carrying passengers) there is usually little debate as to their fate. Abandoned elevateds are a rare sight indeed.
In a some cases, however, elevated railroads were not built over a street, but next to it, or in between streets. Examples of this type of construction include some active lines, such as the Park Avenue Viaduct in New York, the Reading Viaduct in Philadelphia and various elevated lines in Chicago (the Red Line north of the Loop and the Blue Line east of Logan Square). Quite often, however, segments of urban, elevated lines have been abandoned, for various reasons: a new at-grade or (more often) underground segment opened, their need was made redundant by a parallel line, or the need they served ceased to exist. Once this occurs, cities are left with long, grade separated rights-of-way, and no clear procedure for what to do with them.
Urban viaducts are often seen as a blight, and while they do represent significant infrastructure, there is often pressure to tear them down. In Boston, no one could wait to get rid of the Central Artery—there was almost no discussion of keeping it for any reason. Although it could have been used as an elevated park or a means to connect North and South Stations, consensus was to remove it and reconnect the city to the waterfront. This was likely the correct approach; the structure was close to 100 feet wide and ran between the city and the harbor, casting an ominous shadow.
In some cases, however, disused structures are less abhorred and there is not such swift pressure to demolish them. This, in particular, is the case with railroad structures. Few, if any, highway structures in cities are less than six lanes wide—if you are going to bother building an elevated highway and the various accoutrements which go with it (exits, entrances, underpasses, and such), it makes little sense to build it as a two-lane roadway. A two lane roadway can not handle much traffic, and the marginal cost of adding a few extra lanes is relatively small. Thus, highway bridges tend to be at least six lanes wide, and with shoulders, barriers and supports, they are often 100 feet wide (add a couple of exit ramps and they are even more intrusive). Furthermore, because the roadways need to be accessed from below, these structures are usually built at a minimum height above other streets, often providing less than 20 feet of clearance. Thus, highway structures tend to create large and dark spaces underneath, which are almost universally disliked.
Railroad structures, however, often are built differently. Height is less of a consideration, although elevated structures are usually not built any higher than necessary. However, width is much less of an issue. Highways need to be built to a considerable width because the capacity of a highway lane is only about 2000 persons per hour. One railroad track, however, can carry ten times that many people (trains carrying 1000 passengers and operating at three minute headways are commonplace), so in most cases, no more than two tracks are needed. In a few cases, three tracks are built to allow for extra capacity, and sometimes even four—although since the entirety of Grand Central Station can be served by four tracks, wider structures are rarely necessary. And since railroads don't need breakdown lanes, exit ramps or barriers, elevated railroad structures are rarely wider than about 40 feet, and often only 20 feet from side to side. These structures are not as often seen as the "Chinese Walls" that highways (or railroads built entirely on fill) are compared to, and therefore not universally torn down when they are no longer in use.
While aerial structures have been abandoned for some time, there is not yet a definitive protocol for what to do with them. Some, of course, are torn down and, often, the rights of way are used for new structures, all but obliterating the previous use (except to the well-honed eye). For instance, the CTA in Chicago demolished several short elevated segments, such as the Humbolt Park Line (the only visible traces of which lie in buildings which end suspiciously short of nearby alleys) and the north end of the Paulina Connector (redundant once the State Street Subway was built), which is only visible where the structure is still used for railroad signals.
More recent closures, however, have not necessarily been followed with demolition. As cities have transformed, planners and residents have realized that there is potential to use old viaducts to create unique urban spaces. Demolishing such structures often leave narrow and sometimes-bizarre plots of land which are not conducive to new development (especially when they are less than two dozen feet wide), so the land does not have much intrinsic value. However, the structures are often quite sound (having often been overbuilt) and seen as opportunities to bring green space in to the city—without demolishing the structure. The two most significant examples of this type of reuse are the Viaduc des Arts / Promenade Plantee in Paris and the High Line, which very recently opened in New York City. We'll explore both of these in an occasional series.