More than 280,000 homes could be built in unused space above train tracks, tube lines and the overground network in London according to engineering firm WSP's Out of Thin Air – One Year On report.

WSP, who provided the engineering expertise for Renzo Piano's Shard and the London Bridge Station revamp, calculated the amount of unused land above the city's railway systems using Geographic Information System (GIS) technology.

The engineer calculated that 10 per cent of the potential overbuild sites identified in the report could be used, which would be be enough space for 280,000 new homes.

WSP also applied the technology to see how overbuild could unlock development space internationally. They estimated that Melbourne could create an extra 77,400 homes through rail overbuild, 29,160 could be built in Sydney, and Vancouver could find room for an extra 46,033.

Rail overbuild could help alleviate housing shortage

London is currently experiencing a severe housing crisis. Mayor Sadiq Khan announced last year that the city needs at least 66,000 new homes annually to meet the demand, but only 2,917 homes were planned for construction in the second quarter of 2018.

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The report is a follow up a WSP report last year that mooted the idea of overbuilding on railway. The original report estimated that 250,000 homes could be built over rails, but improvements in their methodology found an extra 14 per cent of land with potential.

Collaborating with University College London's (UCL) department of civil, environmental and geomatic engineering, WSP factored in socio-economic and socio-environmental criteria when determining the likelihood of an area getting developed.

Standard factors such as nearby public transport, population density, and places deemed "opportunity areas" by the local authorities were included in the calculations.

Cities need "radical solutions" says WSP

WSP's figure of 280,000 homes assumes that the buildings would be 12 storeys high with 100-square-metre homes. The size of tracks available was also a constraining factor, as viable developments would require a plot of land at least 100 metres long. In London the boroughs with the most overbuild potential are Brent, Ealing and Croydon.

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"As an industry we need to focus on radical solutions to overcome the housing crisis in our capital," said WSP director Bill Price.

"Rail overbuild is not just about creating new homes, it's also about creating new, safe, environmentally-friendly and vibrant communities that such developments can offer."

Architects are proposing schemes above railway lines in increasingly crowded cities all over the world. In America, Rogers Partners and Nelson Byrd Waltz have suggested building an elevated city park over the rail tracks in Atlanta, Georgia, while Australian firm Bates Smart has proposed building a 45,000-seat stadium above Central Station in Sydney.