Once an elevated freight railway track, New York's High Line is now an oasis for pedestrians. It has been so popular that other cities are following suit, with plans to replicate the formula in London. What is the secret of its success?

In 1980 the last freight train ran along the elevated railway line in the Lower West Side of Manhattan. Reportedly, it pulled three boxcars of frozen turkeys.

Almost 20 years later, in August 1999, local architectural enthusiasts Joshua David and Robert Hammond went along to a public meeting to discuss the future of the High Line.

Within months the two New Yorkers - variously described as total amateurs and neighbourhood nobodies - founded the Friends of the High Line, a charity that has gone on to transform the abandoned railway line into a wildly successful new kind of public space - part-beach, part-park, and part-promenade.

Since opening in 2009, the High Line has become New York City's second most visited cultural venue, attracting some four million visitors a year. Through Mr David and Mr Hammond's work, a relic of the 1930s has become the catwalk of 21st Century New York.

Media playback is unsupported on your device Media caption High Line co-founder Robert Hammond visits Bishopsgate Goods Yard

Already cities around the world are interested in learning from New York.

In Shoreditch, east London, the idea of building a new park on top of the old railway arches at the Bishopsgate Goods Yard, abandoned since the mid 1960s, is being considered.

Chicago is proposing to redevelop 2.7 miles (4.3 km) of disused elevated railway line into the Bloomingdale Trail. Its fellow US city Philadelphia is looking at transforming the Reading Viaduct into an elevated linear park. And in Rotterdam, Netherlands, another old elevated track is being considered as a site for a park and shops. The High Line itself echoes Paris' Promenade Plantee, inaugurated in 1993.

And it's hoped that the formula can be repeated on more besides disused railways.

James Corner, the British landscape architect who designed the High Line, is working on the transformation of London's Olympic South Plaza into part of the future Queen Elizabeth Olympic Park. Corner is also working on a proposal to redevelop Liverpool's 1980s Everton Park.

A competition to design London's answer to the High Line has just been won by a project to grow mushrooms in unused mail tunnels under Oxford Street. It's unlikely to be built, but it was this kind of radical thinking that made the High Line a hit.

Walking UK railways Disused railway tracks in the UK have also been turned into rural walkways. The Parkland Walk, north London is built on a line first opened in 1867 as part of Great Northern Railway. The last train ran in 1970 and a linear park opened shortly afterwards, with two walkable sections. Plans to build a dual carriageway in the late 1980s were successfully opposed by locals and environmentalists. It is London's longest local nature reserve at 4.5 miles (7.2km). Monsal Trail and Tunnels, Derbyshire, was originally built in 1863 by Midland Railway, and operated until 1968. The 8.5-mile (13.6km) Monsal Trail reopened for public use in 1981, but four tunnels remained closed for safety reasons. A restoration project to reopen the four tunnels, in 2011, cost £2.25m ($3.6m).

"It's not surprising that other places would imitate it," says architect and broadcaster Maxwell Hutchinson.

"It's a fantastic project, and we're always looking at ways of increasing the amount of green space in our densely-packed cities."

Whether these schemes can repeat the High Line's success and transform industrial mystery meat into filet mignon depends on a host of factors, however.

Key to the New York project's success is the fact that it's both an elevated park and one of the city's finest walkways.

You can amble along its one mile (1.6 km) route taking in views of the Hudson river, Chelsea and the Meatpacking District. It is a place where you can stop on one of the many benches or banks of steps, and sunbathe or gaze over the railings, or through a large picture window at the passing pedestrians or the cars on 10th Avenue.

But it is not just its incredible popularity that has got developers and city officials talking about the High Line, it is also the tonic effect it has had on land values and real estate prices in the area it passes through.

All along the route, prices have shot up. Apartments that were once in the middle of nowhere are now hot property. Fancy hotels such as the Standard now arch over the old railway line.

As a result, there has been an estimated $2bn (£1.25bn) of new economic activity along the route of the High Line.

So the first thing for any would-be imitator to bear in mind is that it took a great deal of cash to make it happen.

"If the project doesn't work financially, it doesn't work," says Mr David.

When he and Mr Hammond made the case for the High Line to New York City, they estimated that given that parks increase the value of nearby properties and thus their taxable value, the High Line could bring in $262m (£164m) in extra tax revenues to the city over a 20-year period.

It seems that their estimate was too conservative. They now reckon that the value to the city in extra tax revenue over a 20-year period will be somewhere in the region of $900m (£563m) - not bad for a project that cost $112m (£70m).

Not only does the city of New York have funds for capital projects, there were a lot of rich and famous people whom Mr David and Mr Hammond were able to approach for help, including actors Kevin Bacon and Edward Norton.

History of the High Line Elevated section of former New York Central Railroad, which was established in 1847

The High Line opened to trains in 1934

Last freight train ran in 1980

Disused for nearly 30 years

Opened as newly refurbished park in 2009

Joshua David and Robert Hammond co-founded Friends of the High Line, employing architects and designers to build park

High Line now attracts four million visitors per year

Project cost $112m (£70m) to set up and costs $4.5m (£2.8m) per year to run

But it was also the novelty and boldness of the High Line that has contributed to its success. Mr David calls the viaduct "a found object". He and Mr Hammond were both able to see something in it that others could not.

When it came to choosing the architect and landscape architects for the project, they went for something unusual. They opted for a team that hadn't built much but who appreciated both the architectural qualities of the rusting railway bridges and the need to make the greenery in the park distinctive.

The winning team included the Dutch garden designer Piet Oudolf, whose choice of grasses and perennials are not only fashionably "wild" but are also in keeping with the wilderness that grew up after the line's abandonment in 1980.

Mr David and Mr Hammond aimed high, spending money on publicity in Grand Central Station, hiring a Washington DC lobbyist, and - as they like to say themselves - throwing parties in style.

But above all, it helped that the project was inclusive. The High Line is a New York City park and so open to everyone for free.

The pair started as community activists and, despite the High Line's wealthy patrons, feel the need to root the High Line in the mix of communities through which it passes.

Mr David says this was deliberate: "As a tourist you want to go where the locals go."

Inclusiveness comes at a price, however. The problem with making the High Line a public park is that, as Mr David and Mr Hammond ruefully admit, they haven't managed to capture much of the value that it has generated for others.

Property developers have made far more money out of the High Line than its own creators, who now have to find $4.5m (£2.8m) in charitable donations every year to keep it open.

There is another downside, too. Such developments mean disused lines cannot be once again brought back into their original use.

"The Docklands Light Railway [in east London] runs on viaducts that were long abandoned," says Hutchinson. "We must be careful not to prejudice potential future railways."

But if it stays true to the spirit of the original, the next High Line won't be a high line at all - it will be something else, something very unexpected.