They've got one in New York. Paris arguably had the first. And Tokyo's has been a huge hipster success. So it was, perhaps, inevitable that China would get a high line of its own.
Architectural firm Hassell has won a competition to design a 1.6-kilometer elevated walkway that would transform Shenzhen's Qianhai business district.
Dubbed the Silk Road Corridor, the winning design proposes connecting buildings and city streets in the area with a multi-level green corridor that would feature a jogging and cycling path, as well as outdoor dining areas and events venues.
It would be the new "social heart" of the district -- a "mixed cultural, recreational and education precinct" within a large park setting, says Dennis Ho, principal and project leader at Hassell's office in Hong Kong.
Beginning of high ambitions
The New York High Line may be the world's most-famous elevated walkway, sparking a global trend for such projects. But when it opened in 2009 it was not technically anything new. New York's success was in expertly refining an existing concept. And giving it a catchy name.
Paris, in fact, created the first elevated walkway on a disused rail line -- aka high line -- back in 1993. The Promenade Plant-e tracks the former Vincennes railway, which was constructed in 1859 and last saw a working train in 1969.
Verdant and visually stunning, it was groundbreaking. But tucked away in the city's 12th arrondissement, most tourists have never heard of it.
Now its New York successor has sparked a global craze for elevated rails-to-trails projects that seems unstoppable.
The Seoullo 7017 opened in 2017 in the Korean capital. Designed by Rotterdam-based architects MVRDV, its name is a portmanteau of 2017 and 1970, the latter being the year the mile-long stretch of overpass on which it stands originally opened.
The walkway connects Seoul Station to Namdaemun Market, and beyond, and features almost 25,000 trees, shrubs and flowers in cylindrical planters which light up blue at night.
Critics, however, have said the "bleak" viaduct fails to shield visitors from the elements and contains too much concrete and too little greenery.
The Seoul creation arrives two years after 2015, a killer year which saw the opening of the Log Road Daikanyama in Tokyo, Japan; the Bloomingdale Trail in Chicago, United States; the Luchtsingel in Rotterdam, Netherlands; and the Goods Line in Sydney, Australia.
Each city has its own take on the concept.
For the Log Road Daikanyama, the focus is as much on shops and cafes as oaks and shrubs, showcasing Japan's flourishing craft beer scene -- one of the reasons Vogue magazine dubbed the area "the Brooklyn of Tokyo".
Meanwhile, the Luchtsingel was the "world's first crowdfunded public infrastructure project," with individuals paying to customize planks along the trail for -25 ($28). A -4 million ($4.5 million) municipal grant also helped this blazing yellow path connect three areas that had been impassable by foot or bike.
'I wanna be a part of it'
So what exactly made New York's High Line so franchisable?
Firstly, it had a perfect location. Perched on Manhattan's Far West Side, it offered an elevated view of Midtown skyscrapers, the Hudson River, West Chelsea and the Meat Packing district. All in an area that had traditionally been industrial, relatively affordable (for Manhattan) and overlooked by tourists.
For charm, landscape architecture firm James Corner Field Operations kept the rusting freight rail tracks and plants were encouraged to grow among them. It was both fresh and urban.
Today, a catwalk for joggers, commuters and, increasingly, tourists, since 2014 it has attracted 20 million visitors.
Professor Ray Bromley, director of globalization studies at the University at Albany, New York, tells CNN he believes it was the "extreme contrast" between the horizontal path of the walkway and the striking verticals of the skyscrapers that made the High Line so successful.
That juxtaposition, of course, had worked once before in New York.
"When Central Park was developed in Manhattan in the late 19th century, people started to really appreciate this extraordinary contrast between the buildings and the green space," he explains.
Bromley notes that after Central Park "the buildings with the highest real estate value became those that overlooked the green space. And they still are."
High Line, high prices
The Manhattan High Line's effect on nearby real estate no doubt also caught city planners' attention.
While it cost $187 million and took three years to build, New York recouped construction costs within a year, according to a study by Michael Levere, a PhD candidate in economics at the University of California San Diego, thanks to an accompanying boost in property taxes triggered by the surge of real estate development in the area.
The value of real estate around its first section is now more than double that of properties just a block away.
Chelsea warehouses have been converted into upscale lofts, and new developments by "starchitects" abound, including the last apartment building by the late British designer Zaha Hadid, condos by Norman Foster (551W21 boasts a 61ft sky pool) and 505 West 19th Street, a block by Danish designer Thomas Juul-Hansen, which has slanted windows to protect against nosy passers-by.
James Howard Kunstler, an American social critic and author of "The Geography of Nowhere," tells CNN he believes the High Line has become the most potent form of downtown regeneration "because the despotic presence of cars and trucks degraded the quality of the most ubiquitous form of urban public space, the street".
Indeed, the gentrifying effect has not been limited to New York.
In Chicago, around The Bloomingdale Trail, an elevated railroad that brings greenery to a neighborhood which previously had the smallest amount of open space per capita in that city, property prices climbed more than 7% in three months -- before it had even opened.
Low points of the High Line?
Adapting New York's model, however, is not a guarantee for success, according to urban theorist Christoph Lindner.
"In many cities -- from London to Bangkok -- there are attempts to replicate the High Line. The majority of them tend to stall and turn out not to be economically viable. Most don't get off the ground," the editor of "Deconstructing the High Line: Postindustrial Urbanism and the Rise of the Elevated Park" tells CNN.
Among the non-starters are the Chapultepec Cultural Corridor in Mexico City and London's Garden Bridge. The Hofbogen in Rotterdam has been mired in financial woes for years, while the PetiteCeinture in Paris appears to have been put on ice, in favor of building houses.
And even when it comes to those that do succeed, not everyone is happy.
"The Manhattan High Line unlocked an explosion of gentrification," says Lindner, "or even hyper-gentrification -- where the rich have been replaced by the super-rich."
There are concerns that the proposed QueensWay in Queens, New York, which plans to up-cycle a stretch of abandoned Long Island railway, will have a similar effect.
"What's very interesting is the level of pushback from the local community," adds Lindner, referring to protests about the loss of privacy in the area if tourists stream in and potential gentrification.
The local Woodhaven Residents' Block Association has said: "We believe that leaving the abandoned rail line alone is the best way to satisfy the needs and desires of as many residents as possible."
From high to low
Lindner predicts that "we're going to see a saturation of this concept quite soon".
But for now that doesn't seem to be the case. New projects in the pipeline include the Camden High Line in north London, Singapore's Green Corridor and Philadelphia's Reading Viaduct.
Sharon Zukin, a professor of sociology at Brooklyn College and author of "Naked City," says the concept has become so popular because it satisfies two important criteria for modern cities: the desire to rejuvenate "the industrial detritus of the 19th century" and the need to cater to the growing "experience economy."
The rails-to-trails concept offers "a rare way of experiencing being in the city."
On New York's Lower East Side that concept is being taken one step further by the LowLine -- if built, it will be the world's first underground park.
Located in a one-acre former tram terminal abandoned since 1948, when the service was discontinued, it would rely on "solar collection dishes" above ground to channel light beneath.
"The novelty value is going to wear off," asserts Lindner. But, he insists, "the general trend of adaptive reuse will continue."
"The pressure on cities, particularly when it comes to creating green space, is so intense that we have to continue to adapt what we've got. We can't just keep expanding."
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