Network Rail and contractor ARQ (AmcoGiffen, REL and QTS) have completed a pioneering upgrade to the overhead line equipment and power supply in the UK’s Severn Tunnel.
The 7km tunnel runs beneath the Severn Estuary and is one of the most challenging environments on the UK rail network.
Since electrification, a combination of saltwater and moisture has accelerated wear on overhead line equipment through bi-metallic corrosion, where different metals react in wet, salty conditions and deteriorate more quickly.
To address the issue, Network Rail developed an overhead line system specifically for the tunnel’s harsh environment. The simplified design uses modified bridge arms and a continuous copper contact wire to create a more resilient and reliable railway while reducing future maintenance requirements.
Following a successful trial on the Cardiff-bound line in 2025, Network Rail and ARQ have now completed the installation on the Bristol-bound line, delivering the first large-scale deployment of the system.
The project included replacing 838 overhead line support arms; installing 14km of new wiring; installing more than 7km of single copper contact wire supported by modified bridge arms; and removing and replacing 7.2km of ageing equipment throughout the tunnel.
Around 250 engineers were deployed on the project. Work was carried out around the clock and completed in 16 days.
Nick Millington, route director at Network Rail Wales and Borders, saidcompleting the upgrade was a significant achievement for everyone involved.
“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,” he said.
“Investing in innovative solutions like this helps us build a more resilient and reliable railway for the future.”
ARQ operations director Vinny O’Holloran saidthe engineering and delivery team had worked tirelessly.
“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,” he said.
