Once upon a time, there was an elevated railroad known as the High Line. From 1934 it hurtled through New York's West Side, allowing freight trains to sweep meat, milk and manufactured goods from Manhattan's ports into the second floors of the meatpacking district warehouses. But as industry declined in the city, the railroad fell out of use. A train last used its tracks in 1980, its rails ran into disrepair and its sleepers, well, went to sleep.
That isn't the end of the story. The High Line's tracks, once a rusty scar marring the Big Apple's streets – became overpowered with weeds and seeds spiralling from the sky. Locals, enamoured of the serene individualism of this secret space, launched a campaign to convert the High Line into a park. In 2004, the city's authorities granted them £50m to do so. A serious planting operation later, sprouting everything from meadow sage to switchgrass, black-eyed susans to echinacea, the park attracts 25,000 visitors a day, boosting property prices nearby.
In London, the Greater London Authority estimates that 38 per cent of the city is green space. The city's mayor, Boris Johnson, hopes to grow that number by 5 per cent annually. Given the the tightness of urban space and inner-city land prices, he would do well to invest heavily in elevated and rooftop parks and gardens. These act as a city's lungs, natural insulation tools which provide habitats for insects and birds.
Opening a year ago this month, the High Line's success has inspired a generation of parks in America – and beyond. St Louis, Atlanta, Chicago, Minneapolis, Virginia, the Florida Keys, Oakland and Dallas are all mooting similar schemes. Paris has an elevated park, Mumbai has elevated walkways linking green spaces, Toronto wants to muddy its shoes, as does Shenzhen in China. The British architecture firm Terry Farrell and Partners is working on converting a viaduct near the former Bishopsgate goods yard in London's Shoreditch into an elevated park. Want your city on the map? Get second-storey foliage.
"We're reconstituting the fabric of the city," says Craig Schwitter, of engineering firm Buro Happold's New York office, who worked on the High Line project along with architect James Corner. "We were trying to knit something in with what was already there. Rather than other examples of elevated parks– which might be on the top of buildings, for example – it's more of a filigree piece of urban infrastructure. We've taken on an industrial relic and improved it in quite a fragile way. We've stripped down the steel and reused what was there. When you mention infrastructure to most people, they think you have to tear it down and then build it up again. We wanted to avoid that".
Elevated parks are mostly built on raised former rail lines, or industrial buildings. Many urban green spaces, elevated or otherwise, offer reinterpretations of different, former uses. London's Royal Parks and Paris's Jardin du Luxembourg were originally used solely for the recreation of the British and French royal families before opening to the public, for example. Urban areas are in constant flux, whether it is the post-industrial decline of Germany's Ruhr Valley or the replacement of Manhattan's manufacturing with service industries.
On the High Line, pebble-dashed concrete walkways swing from side to side across the former railway's surface, melding pedestrians' paths with planting embedded in gravel mulch. Some of the original tracks are still visible. There are some 210 species now planted on an area accessible by five stairwells, though there are plans to expand. While its designers say it is cheaper than building a raised park from scratch – though probably more expensive than a conventional park to maintain, and build – there is no way an original project of this nature, which cuts through entire city blocks, would be granted planning permission. Maintaining such city-centre bolt-holes throws up fresh challenges. Special crews use a crane to lift equipment for special events and public programming. Gardeners use a small bike with a basket carrying plants and gardening materials from one end to the other.
While the Promenade Plantée – a park constructed on a former railway viaduct – has been pacifying Gallic flâneurs since 2000, other schemes in the US hope to ride in on the High Line's popularity, again using the existence of original infrastructure to cut down on costs. Alexander Garvin, a New-York-based urban planner, intends to use the principles of the "Emerald Necklace" – a circular chain of parks in Boston that combine a drainage system, a recreation facility, a transit line and a framework for real estate development – to Atlanta, where a ring of freight lines encircle its downtown district. Garvin has proposed a 20-mile transit line combined with a 23-mile trail, with 13 parks as jewels on the necklace. Only parts of the freight line are elevated, meaning the height of any parks will be simply a by-product of the existing railway. "All of these conversions are very opportunistic," he says. "Property is no longer being used by its former owner and it no longer functions properly. You get left with these post-industrial relics."
Striking out on its own is a proposed redevelopment of Manhattan's Pier 57 by New York designers LOT-EK. The hope is to overhaul the interior of this former industrial building using shipping containers, finessing the building's summit with a green park. "It could help its insulation," says project architect Giuseppe Lignano. "So why not make it something the entire public can enjoy? Plus it provides a vantage point above the water. It is one of the most unobstructed views in the city."
In Mumbai, city authorities are building a series of elevated walkways to ease congested pavements. The first opened last year and has been nicknamed the "Yellow Caterpillar". It connects commuters pouring out of the city's Bandra Station to a nearby city park. Meanwhile. New-York-based architects Work have won a competition commissioned by Shenzhen's Planning Bureau to build a series of "figure-8" style structures in one of the city's busiest streets to serve as bridges and raised parkland.
Not everyone is in favour of raised public spaces. In Mumbai, retailers have complained they are losing business because of diverted footfall while residents are moaning that the skywalks are blocking their views, allowing pedestrians to peek into private homes. Farrells' Shoreditch plans are facing opposition from residents who say that the raised park would not connect two urban areas, as in Paris or New York. Tower Hamlets Council's proposals, say their opponents – while still in the very early stages of planning – would be be largely inaccessible to those living closest to them.
In short, therefore, while elevated parks are the must-have green space accessory for any city, they should not be applied in a "one-size fits all capacity". Certainly not before any important stakeholders have been canvassed. Moving forward, it might be best for city residents to gird their loins. Everything from floating parks powered by water power to grid-like skyscrapers with huge, sweeping walls of jade and emerald creepers have been visualised (GRO Architects' very cool floating parks look like a cross between a yellow Lotus Esprit and a chunky Frisbee). Things can only get greener. The High Line's success points in the right direction. The future, it seems, is vertical.