Network Rail completes pioneering £23m Severn Tunnel modernisation project

Train services have resumed through the Severn Tunnel following the completion of a major engineering programme that has introduced a new overhead line system designed specifically for one of the most challenging railway environments in the UK.

Network Rail invested £23 million in the project, which was delivered during a 16-day railway closure by contractor ARQ, a joint venture comprising AmcoGiffen, REL and QTS. The works focused on improving the long-term reliability of the electrified route connecting South Wales and England.

Around 250 engineers worked continuously throughout the closure period to install a newly developed overhead line arrangement inside the 4.35-mile tunnel beneath the Severn Estuary. The route reopened to rail traffic on 9 June.

Innovative approach to a long-standing challenge

The Severn Tunnel presents unique maintenance challenges due to the presence of saltwater seepage, high humidity and a corrosive atmosphere. These conditions have historically accelerated deterioration of electrification equipment, particularly through bi-metallic corrosion, where different metals react in damp, saline environments.

To address the issue, Network Rail developed a simplified overhead line design tailored specifically to withstand the tunnel’s harsh conditions. The new system incorporates modified bridge arms and a continuous copper contact wire intended to reduce component wear and lower future maintenance requirements.

Following successful testing on one tunnel bore in 2025, the solution has now been rolled out across the second line, marking the first large-scale implementation of the technology.

The programme included the replacement of 838 overhead line support structures, installation of 14km of new wiring and more than 7km of continuous copper contact wire. Engineers also removed and renewed over 7km of ageing infrastructure throughout the tunnel.

The completed upgrade is expected to enhance reliability for both passenger and freight services using the route, which forms a critical transport corridor between Wales and England.

Nick Millington, Route Director for Network Rail Wales and Borders, said:

“The Severn Tunnel is one of the most challenging environments anywhere on the UK rail network, so completing this upgrade is a significant achievement for everyone involved.

“Following a successful trial last year, we’ve now delivered a world-first engineering solution at scale, designed specifically to withstand the uniquely harsh conditions inside the tunnel.

This is the result of years of innovation, planning and collaboration between Network Rail and our industry partners to solve a long-standing challenge on a vital rail link between South Wales and England.

“Investing in innovative solutions like this helps us build a more resilient and reliable railway for the future. By reducing the impact of corrosion and the need for maintenance, this new system will help keep passengers and freight moving on a route that plays a crucial role in connecting communities, supporting businesses and driving economic growth on both sides of the border.

“I’m incredibly proud of the hundreds of colleagues and contractors who worked around the clock to deliver this complex upgrade safely and successfully. I’d also like to thank passengers, freight operators and local communities for their patience and understanding while we carried out this essential work.”

Vinny O’Holloran, Operations Director at ARQ, praised the efforts of the delivery team:

“It’s been great to see the level of effort and commitment of our 250 strong engineering and delivery team working tirelessly 24/7 for the past 16 days to deliver this vital overhead line renewal in the Severn Tunnel safely and efficiently.

“From working in the extreme heat, to overcoming the uniquely challenging conditions within the Severn Tunnel, our teams have shown outstanding resilience, skill and professionalism to successfully deliver this innovative, world-first Overhead Line system.

“These essential works will help to create a stronger, more resilient rail infrastructure between Wales and England reducing future disruption to passengers and freight.”

Wider rail improvements completed during closure

The possession also provided an opportunity to carry out a series of infrastructure upgrades across the Bristol area.

Drainage improvements between Patchway and Pilning included the replacement of almost 200 metres of pipework, refurbishment of a further 700 metres and upgrades to 18 inspection chambers. The work is intended to reduce flooding issues that have previously affected services through the Patchway tunnel.

Track renewal schemes were also undertaken, with more than 1.5km of railway track replaced. Inside Patchway tunnel, crews installed 742 sleepers and 1,800 tonnes of ballast, while a further kilometre of track between Filton and Patchway was renewed using more than 2,000 sleepers and 3,000 tonnes of ballast. Additional work was completed at Filton West Curve and the Filton West Chord ahead of the planned return of passenger services to the Henbury line later this year.

Progress was also made at the new Bristol Brabazon station, where a nine-section footbridge was lifted into place and assembled during the closure. Construction of the station continues ahead of its anticipated opening this autumn.

Marcus Deegan, GWR’s Station Manager for Bristol Parkway and Swindon, said:

“We’re pleased that trains are now running again between Bristol and South Wales, and we’d like to thank customers for their patience while this important work has been carried out.

“These improvement works help support the reliability of trains using the Severn Tunnel route for years to come, better supporting the local and national economy.”

The completion of the Severn Tunnel works represents a major milestone in improving the resilience of one of Britain’s most important rail corridors while supporting future passenger and freight growth across the Great Western network.