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Use rail overbuild to unlock capacity, says WSP

Kensington rail overbuild

Building apartments over rail tracks could solve London’s housing needs but high engineering costs could prove to be a deterrent, according to research by WSP.

In 2012 Network Rail tasked WSP with researching the feasibility of building new developments over railways, and the results are detailed in a report called Out of Thin Air.

By using sections of track where there are no breaks due to tunnels, roads or bridges, and there is 10m of land either side, WSP estimates 12 storey buildings could house 100m² apartments over rail lines. Developing 10% of the available space in TfL’s zones one to six could lead to 250,969 new homes.

Available rail lines for development

Available rail lines for development

Source: WSP

Available rail lines for development in London

Recent developments in tackling structure-borne noise and vibration from train movements means that mitigation methods, such as separating the superstructure from the base of a building with springs, are now much cheaper. More digital innovation also allows a more tailored approach and so less costly engineering.

But costs surrounding concrete rail boxes enveloping the rail corridor below a new building could prove a barrier to widespread development.

“We have to be more creative in using existing space in what remains a relatively low-rise city. The air rights above rail tracks present an unrealised but significant opportunity to build more new homes on brownfield land,” said WSP director Bill Price, stressing that building over rail tracks is not new: Chelsea Football Club’s proposed Stamford Bridge stadium will sit across rail track.

“In some parts of London rail lines act as accidental segregators. By ‘decking’ over these lines, such as the proposed regeneration west of Earls Court underground station, we can join together sites to unlock an even higher number of new homes and create new vibrant communities.”

Creating a concrete box enveloping rail lines below a new development is crucial to overbuilding, as it supports the structure while allowing the railway to function as normal. Rail box walls and decking should be made of solid concrete to minimise noise impacts and allow offsite manufacturing.

But high prices associated with decking could still be off-putting to developers. Using short-span beams with minimal depths, solid walls instead of columns and simple foundations could all help make overbuilding more attractive.

The rail box is the responsibility of the rail authority and so early collaboration between the designer, developer and rail authorities is vital for success of a scheme, avoiding delays and costly overruns.

Last week mayor of London Sadiq Khan revealed London needs to build 66,000 new homes every year to bridge the gap in demand, roughly double the current rate of homebuilding.

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