WORK started today (31) on the HS2 railway’s first and longest viaduct.

An enormous 700-tonne bridge-building machine was shown off at an event attended by HS2 Minister Andrew Stephenson.

Stretching for more than two miles across a series of lakes and waterways between Hillingdon and the M25, the Colne Valley Viaduct will also be the longest railway bridge in the UK.

Known as a ‘launching girder’, the 160-metre long bridge-building machine is the only one of its kind in the country and will be used to lift into position the giant concrete deck segments that form the viaduct’s arches.

Once each section is complete, the machine will inch itself forward into position to build the next stage.

A total of 56 piers, each weighing around 370 tonnes, are being constructed along the Colne Valley, with the girder moving from one pier to the next.

A total of one thousand deck segments will be needed, with each one weighing up to 140 tonnes.

To allow for the gentle curves of the viaduct as it crosses the valley, all the segments are slightly different in shape and made on site.

The mammoth pre-cast factory, visible from the M25, is larger than the Royal Albert Hall.

Once construction is complete, the factory and surrounding buildings will be removed and the whole area between the viaduct and the Chiltern tunnel transformed into an area of chalk grassland and woodland.

The design of the Colne Valley Viaduct was inspired by the flight of a stone skipping across the water, with a series of spans carrying the railway above the surface of the River Colne, Grand Union Canal and adjoining lakes.