Just a few blocks north of Philadelphia’s Center City, with its immaculate grid designed by the city’s founder, William Penn, the landscape turns hardscrabble. The slick, glass-skinned office buildings give way to visions of earlier eras of industry and urban planning: old brick warehouses and granaries, the Vine Street Expressway — a neighborhood-wrecking classic conceived during the height of midcentury urban renewal — as well as several heavily trafficked multilane streets. Running below and above this jumble are the remains of an old rail line, a brick-vaulted tunnel that emerges into an open-air, walled and grassy corridor. It climbs slowly past the Art Deco Terminal Commerce Building — once one of the largest warehouses in the Northeast, today one of the region’s largest data-storage facilities — into a rusted elevated rail line, called the Reading Viaduct.

A former conduit for coal and other freight, the viaduct exudes the late-capitalist poetry of urban neglect. You can follow a path through waist-high weeds hacked by enterprising city explorers, who cut holes in fences emblazoned with NO TRESPASSING notices to gaze nostalgically at ghostly ironworks signage, or at the reddish girders of a defunct energy-transfer station. Paulownia trees dot the viaduct, another relic of long-gone global commerce: The tree’s pistachio-shell-shaped seeds were used as packing material for Chinese porcelain in the 19th century, falling out of crates to seed and sprout along railways around the world.

Soon this sprawling postindustrial vision, too, will be part of the past. Ground was broken last month on the Rail Park, a rehabilitation project to turn the Reading Viaduct into an elevated park. It follows a recent flurry of similar parks: the Bloomingdale Trail in Chicago, the Atlanta BeltLine and, of course, the wildly successful High Line in Chelsea in Manhattan. All drew inspiration from the Promenade Plantée in Paris, completed in 1993, the first conversion of an elevated railway into a pedestrian park.

If the new railway parks have revitalized the infrastructure of the early-20th-century industrial economy, the process of “capping,” building green spaces over freeways, turns urban expressways, the hallmark of midcentury planning, into vital links among communities. In 2012, Klyde Warren Park capped the Woodall Rodgers freeway in Dallas, bringing together northern and southern parts of the city, while Boston’s notorious Big Dig project was covered by the Rose Kennedy Greenway in 2008. Monuments to ways of life and work that we no longer require are being converted, one by one, into promenades and playgrounds, changing what we think cities are for and how they ought to be used.

This model of urban design has come to be known as landscape urbanism. One of its primary theoreticians, Charles Waldheim, a professor at Harvard’s Graduate School of Design, traces its roots to Frederick Law Olmsted. In his book “Landscape as Urbanism,” Waldheim notes that Olmsted was one of the earliest figures to call himself a landscape architect (though it was a term that Olmsted considered inadequate), and that he was full of visions for integrating landscape and urban life. But Olmsted’s Central Park, for example, is a classic instance of placing nature in opposition to the city: it’s meant to be a balm, an oasis; in rocky, deliberately overgrown sections like the Ramble it mimics the wilderness a city must exorcise and destroy.

The landscape urbanist, by contrast, sees the decline of the industrial city as prompting a variety of opportunities for naturalism. In cities dominated by service economies, Waldheim argues, landscape urbanism can clean up the industrial economy by reintegrating it with the natural world. In so doing, it creates entirely new urban experiences: The High Line makes it possible to saunter in the air, among apartments and offices, at a much slower pace than at street level. As James Corner, a designer of the High Line, once wrote, “The visitor becomes as much a performer as viewer, more deeply engaged in participating in the theatricality of urban life — the promenade as elevated catwalk, urban stage and social condenser.” The experience is nonetheless limited and directed, a kind of soft coercion. You aren’t encouraged to linger on the High Line: with its crush of tourists, it’s more a Manhattan-themed ride than a park.

The new parks aren’t just oases, then; they emphasize the intertwining of landscape and industry. It’s no coincidence that landscape urbanists are beguiled by Detroit, a crumbling monument to what is sometimes called Fordism — after Henry Ford’s mass-production-mass-consumption model — a paradise of dereliction reclaimed by nature. A “formerly urban” space, in the terminology of Waldheim and his peers, it offers little for traditional architecture but everything for an urbanist trained to see value in rescuing the decaying built environment. Detroit, for a landscape urbanist, is a symbol for processes taking place in cities everywhere: once-bustling ports shrunk by the advent of the shipping container, hulking buildings abandoned as low-wage economies replace high-wage ones. In the Qianhai area of Shenzhen, China, Corner designed a new district that reclaims industrial land, repurposing the tributaries of several channels recently used for drainage as “waterfront.”

Similarly, defunct rail lines are sometimes meant not to be repurposed but to be re-embraced. Both the Atlanta BeltLine and Chicago’s Bloomingdale Trail take transportation infrastructure and turn it into newer transportation infrastructure. The BeltLine connects disparate neighborhoods by bike (and soon, streetcar) in a city that is a byword for sprawl, while the Bloomingdale Trail becomes a raised express lane for cyclists and walkers. Klyde Warren — a well used if anodyne public park, filled routinely with yoga classes — connects Dallas’s Uptown and Downtown neighborhoods, previously walled off by the freeway. It’s now become possible, as I discovered, to walk through a generous portion of Dallas, something unimaginable just a few years ago. It’s post-post-industrial planning — trying to make whole the freeway-carved cities that date from an era when car-centric planning was in vogue.

Yet for all its power to bring communities together, repurposing freeways and old industry can go hand in hand with gentrification, increasing inequality and displacement. The effect has been profound in formerly down-at-heel spots along Atlanta’s 22-mile BeltLine, where old industrial sites have been converted into new market-rate and luxury housing. Philadelphia’s Rail Park is already being promoted by local landlords as a selling point for new housing in the surrounding Callowhill neighborhood (a section is currently being rebranded as the Spring Arts District). In both instances, it’s not that the parks created demand where none existed but that they drove, or will drive, already ongoing development. The High Line did not suddenly draw attention to an otherwise unknown Meatpacking District (though it certainly drove interest in Hudson Yards). Still, even boosters concede that these projects have the potential to displace longtime residents. Ryan Gravel, whose master’s thesis in city planning and architecture became the basis for the BeltLine, eventually resigned from the board of the Atlanta BeltLine Partnership. Gravel wanted more focus on and subsidies for affordable housing.

Whom are these new parks for? The question seems moot: Public parks, of any kind, are inherently democratic. But projects like the High Line aren’t solely public in their financing. Many of them are essentially highly cultivated promenades; there’s little room for play, for picnicking. This wouldn’t matter, ultimately, if it weren’t for the fact that these parks consume far more resources than other urban parks and receive far more attention. The High Line’s 2.8 acres have squadrons of gardeners tending to the plant life, to say nothing of custodians and bathroom attendants. In 2013, as it was being completed, the High Line received more money from New York City than nearly every other park. Some of the profits from park concessions at the High Line go to the private nonprofit that oversees the park rather than to the city. In October, Pennsylvania state funds were released to help the nonprofit Friends of the Rail Park complete the first phase of construction in Philadelphia.

It’s instructive to imagine what this model would look like applied to transit. Would it be legitimate to have a Friends of the New York City Subway that raises millions of dollars in private funding in order to retrofit a section of subway line — which, newly cleaned up and rat-free, would spur development in the surrounding area — and then receive more in taxpayer dollars than any other subway line? Why not privatize everything?

We haven’t gotten there yet. Parks still subsist largely on public funds. But places like Central Park and Bryant Park look better than they did in the 1970s in part because they get a significant boost from partnerships and private fund-raising. The High Line is perhaps now the greatest outdoor corporate event space in New York City. The Friends of Van Cortlandt Park in the Bronx have a tougher time. What is so attractive about the parks, in other words, is also what’s wrong with them. The new public parks give form to cities misshapen by abandoned industry, but threaten to bring into being a novel form of inequality in cities already rife with it. They exude the priorities of a new Gilded Age, even as they cover up the eyesores of an old one. ♦

Nikil Saval is an editor at n+1 and the author of “Cubed: A Secret History of the Workplace.”