The viaduct will be 2.1 miles long and cross the Colne Valley Regional Park in Denham. It will pass over the Hillingdon Outdoor Activity Centre (HOAC), which will be relocated to Broadwater Lake, the Grand Union Canal, Savay Lake and Moorhall Road. It then crosses several lakes in the Broadwater Lake Reserve as well as the River Colne before crossing the A412 North Orbital Road. The route leaves the viaduct onto the newly constructed West Hyde Embankment on its way to the Chiltern Tunnel. Denham Water-ski Club has had to build a new clubhouse on the south side of the lake.

The Colne Valley Trail (part of the London Loop) will remain open apart from a short time when the section is constructed overhead. It will be temporarily diverted via a pontoon in the canal before being reinstated along its original route. The Grand Union Canal towpath will remain open throughout construction apart from a short time when the viaduct is constructed overhead. PRoW Den 3 (Old Shire Lane) will be diverted during construction and then reinstated under the viaduct.

There will be a number of construction compounds along the viaduct route. Access to these compounds will be via a temporary haul road which follows the route across the lakes on a series of jetties then on to the South Tunnel Portal compound and then the slip roads to the M25.

The viaduct will be supported by 56 piers and two abutments. The deck will consist of 1,000 sections made on site in the viaduct precast factory and put in place using a launching girder named Dominique. The viaduct deck is due to be completed by the end of 2024 with full completion by May 2025.

NB. The viaduct was completed at the end of August 2024 apart from one section which was installed at a ‘topping out’ ceremony on September 5th. It is Britain’s longest railway bridge.