HomeInfrastructureElectrificationElectric train services from London to Cardiff begin as the Severn tunnel...

Electric train services from London to Cardiff begin as the Severn tunnel is electrified


The electrification of the Great Western main line has reached another milestone, as the first all-electric passenger services are now running from London Paddington to Caerdydd Canolog Cardiff Central.

While the first electric passenger service reached Cardiff on 5 January 2020, the bi-mode train had to use its diesel engines through the Severn tunnel. This was due to unforeseen problems with the electrification system inside the tunnel.

In open air, the overhead contact wire is made from copper and is suspended from a catenary system supported by either lineside posts and cantilevers or gantries (portals). The wire is kept under tension to resist the upward force from the train’s pantograph.

In the tunnel, where space is limited, the contact wire is instead clipped into and supported by an extruded aluminium beam that runs the length of the tunnel. This, in turn, is suspended from the roof of the tunnel using insulators, so the high-voltage beam assembly, which is all energised at 25kV, is insulated from the tunnel by both the insulators and the air gap between it and the tunnel lining.

The conductor beam system inside the Severn tunnel.

The aluminium structure of the entire support assembly oxides shortly after assembly by a natural process. That oxide layer is stable and protects the raw aluminium underneath.

However, in the Severn tunnel, engineers found that something was attacking – ‘eating’ – the oxide layer. This exposed the bare aluminium, which then oxidised again. The process kept repeating, slowly consuming the aluminium structure.

One theory was that an anaerobic bacteria was responsible. This lives in wet environments and sometimes gives trouble on oil rigs.

For this reason, the tunnel was deemed unsafe to electrify on a permanent basis.

Network Rail and overhead line manufacturer Furrer+Frey approached the problem in several different ways.

The copper contact wire can just be seen on the underside of the aluminium support beam.

Furrer+Frey replaced the copper conductor with a similar aluminium wire. This removed the possibility of galvanic corrosion occurring between the copper wire and the aluminium supports. Aluminium generally isn’t used for contact wires as it wears slightly more quickly than copper – but only marginally – and has a lower tensile strength for the same cross-sectional area, so it doesn’t tension as well. However, in the tunnel, where the wire is supported along its whole length by the beam, it isn’t under tension anyway.

Network Rail, for its part, ‘dried up’ the wet tunnel. Drip pans were replaced, drainage channels checked and some obvious leak paths re-grouted. As a result, water ingress into the tunnel was reduced, or at least channelled away from the overhead contact system.

The Severn tunnel has always been wet. The Great Spring, uncovered when the tunnel was first dug in 1879, flooded the workings and still has to be pumped out continuously, even today. However, some of the leaks in the tunnel were salt water from the Severn estuary, and having a 25kV electrification system actually running wet with salty water – a good electrolyte – isn’t a great idea!

With the aluminium contact wire installed, and the tunnel noticeably drier and less humid, test trains were run, including a few passenger service trains that went through with their pantographs up, and no more problems were found.

Interestingly, no actual evidence of the mysterious anaerobic bacteria was found. 

As a result, both parties were now confident enough to remove the ban on electric trains, so, on Friday 5 June, electric passenger services ran all the way from London Paddington to Cardiff.

The first electric train to Cardiff, on 5 January 2020, had to run on diesel power through the Severn tunnel.
Mark Langman, Network Rail

Mark Langman, Network Rail’s managing director for Wales and Western, said: “I am absolutely delighted that the Severn Tunnel is now fully electrified, resulting in a seamless rail link for passengers between Cardiff and London Paddington.

“Electrification has reduced journey times between South Wales and London by as much as 15 minutes, and provided an additional 15,000 weekday seats, compared with a year ago, with the possibility of further increasing the number of services and seats from South Wales in the future.

 “It has been a hugely complex task to electrify the tunnel, but I’m thrilled that the final piece of the puzzle is now complete.

 “I would like to thank passengers and lineside neighbours for their patience over the past decade, as we worked to deliver the transformation of this vital railway, and am pleased that they will benefit from these improvements for years to come.”

Ken Skates, Minister for Economy and Transport , Welsh Government.

Ken Skates, Minister for Economy, Transport and North Wales in the Welsh Government, added: “The completion of the electrification works in the Severn Tunnel is welcome news in terms of improving journey times between capitals and delivering towards our decarbonisation agenda for transport. I would like to thank Network Rail for completing this work in what must have been challenging circumstances.

“I hope that the completion of this work can be taken as a strong signal of intent from Network Rail and the UK Government that there is more investment to come to improve journey times and capacity across South Wales in the future, including electrification westwards beyond Cardiff.”

Plans to electrify the line between Cardiff and Swansea, and into Bath and Bristol, have both been postponed for the present.

Looking further forward, Sudbrook pumping station, which extracts the Great Spring water from the tunnel, is also set to be renewed as part of the upgrade of the Severn Tunnel, further improving the reliability of the railway.


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