The HS2 announced the start of the construction of the railway’s first and longest viaduct.

HS2 Ltd launched the giant 700 tonne bridge-building machine, used to lift 1000 dock segments each weighing 140 tonnes.

The new Colne Valley Viaduct, stretching more than two miles (3.4km), is needed to cross lakes and waterways between Hillingdon and the M25.

Once finished, it will be the longest railway bridge in the country.

HS2 Minister, Andrew Stephenson said: “Today, HS2 began construction on what’s set to be Britain’s longest railway viaduct, a landmark moment for HS2 and a feat of British engineering, taking the HS2 line from London, and into Hertfordshire and Buckinghamshire.  

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“Infrastructure is the backbone of HS2 and this viaduct will be integral to delivering faster journeys and an increased capacity rail network.”

The bridge-building machine itself, known as 'launching girder', measures 160 meters, and is the only one of its kind in the UK. 

Bucks Free Press: A CGI depiction of the Colne Valley Viaduct (Image: HS2 Ltd)A CGI depiction of the Colne Valley Viaduct (Image: HS2 Ltd)

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It lifts the heavy concrete deck segments in to place, and once each section is complete, the launching girder can inch itself forward to build the next section of the viaduct. 

HS2 Ltd Chief Executive Mark Thurston welcomed the start of assembly works.

He said: “It is yet another big milestone for HS2 Ltd, as we work to deliver the UK’s new high speed railway.

"Once complete, this record-breaking structure will form a key part of the HS2 railway – helping to deliver better connections across the UK, free up rail capacity on the train network, and offer passengers a zero carbon travel option.

“I’d like to thank all those involved in getting us to this exciting stage and look forward to seeing the whole viaduct come together over the coming years.”