HIGH-speed rail builders HS2 Ltd have released new images of the project's longest viaduct.

When complete, it will span 2.1miles across the Colne Valley from Harefield and Rickmansworth. Work passed the halfway point this autumn.

It is engineered to carry trains speeding between the London and Chiltern Hills tunnels at up to 200 mph.

The landmark structure will also become Britain's longest rail bridge.