By Phil Marsh

Longest UK railway tunnel to be closed for six weeks bringing train alterations between Bristol and South Wales

Published: 28th August 2016

A short history of a long tunnel

The Severn Tunnel will be closed from 12 September to 21 October for electrification work which will allow the Hitachi electric trains to be introduced from next year. Without the closure, the tunnel would have taken an estimated six years to electrify Network Rail (NR) says. At the moment, there have been several short closures allowing NR to carry our preparatory work which has shortened the length of the closure.

Perhaps surprisingly, remnants of the steam age have had to be removed. An estimated four tonnes of soot has been power brushed from the tunnel lining and this has revealed where brickwork repairs were required.

The tunnel is over four miles long and houses two tracks so NR has to equip the tunnel with eight miles of fixed overhead conductor rail rather than the usual overhead wire system. The scale of the tunnel project is huge and will require a huge collection of plant and machinery to carry out the work meaning that the closure was unavoidable. NR says that over 200 engineers will be working round the clock during the closure.

They said:

Paul McMahon, route managing director for Network Rail Wales said: Without a solid six-week closure, it would take engineers up to five years to complete the upgrade, causing long-term disruption for passengers and delaying the new electric trains until 2021.

Dan Tipper, area director for Network Rail Wales, said: “Wales is open to passengers and freight traffic during the upgrade work but we are urging people to plan their journey ahead.

“While this iconic project will result in short-term disruption, there are significant long-term benefits which will come as a result of electrifying the railway to Cardiff by 2019 including faster, more frequent trains and a boost to economic growth in South Wales thanks to better connectivity to and from London, a critical factor for attracting inward investment.

“We would like to thank passengers for their understanding and patience as we complete this essential upgrade."

Tunnel closed but lines into Wales not shut

The direct route between Bristol and South Wales may well be unavailable while the tunnel closure is in place but Newport, Cardiff and westwards will remain rail-connected with trains running via Gloucester and Chepstow plus a rail-replacement bus service. And don’t forget that trains from Portsmouth and Taunton to Cardiff will also be affected.

If you are travelling from Paddington, Reading, Didcot or Swindon to South Wales, you will be diverted over the scenic route via Gloucester with journey times around 30 minutes longer and trains running on an hourly basis with another train each hour running to Bristol Parkway.

Other rail services which normally use the tunnel will be altered

The Porstmouth to Cardiff services will operate between Bristol Parkway and Portsmouth Harbour while services to the Southwest will run from Bristol Parkway to Taunton and trains to Weston-Super-Mare that would normally start/terminate at Bristol Parkway will start from Filton Abbey Wood.

Buses will replace trains between Severn Tunnel Junction and Bristol Parkway and Bristol Parkway and Newport adding around 45 minutes to the normal journey time.

Local training and electrification employment

A training facility at Coleg Y Cymoedd in Nantgarw has been jointly funded by the college and the Welsh Government. It features a life-sized mock-up of the overhead line equipment that will be used in the tunnel.

NR is working in partnership with contractors ABC and AMCO, and supported by McGinley Support Services, to deliver the training that enables engineers to build, dismantle and maintain the overhead power lines and specialist equipment unique to the Severn Tunnel electrification project.

It is the sole such facility in Britain to offer training on this specially designed piece of equipment known as a rigid overhead conductor rail, which is also used on Norwich Trowse swing bridge.

Severn Tunnel history

In 1863, The Great Western Railway decided to build a bridge, estimated to cost £1million over the Severn which would have been two and a half miles long and allow ships of up to 122 feet high to pass beneath it.

The option of a Severn Tunnel was decided on in 1872 when an Act of Parliament was applied for to obtain construction powers. It took 14 years to complete with many setbacks.

The tunnel had to be located 140 feet below the river to avoid a gulley called ‘Shoots’ on the Welsh side which descends 50 feet below the level of the English side. The tunnel is 4.3 miles long, driven by the need for a maximum gradient of 1 in 100 for the railway. The tunnel’s interior measures 24.5 feet high and 26 feet wide.

The tunnel passes through rock, clay, gravel and sand and construction commenced in December 1874. Five years later the miners working uphill on the Welsh side struck what was named the Great Spring. This poured fresh water into the tunnel from an underground reservoir and within a few hours, the tunnel was flooded to a depth of 150 feet in the air shafts.

The Great Spring was cut off by using shields and many pumps were used to drain the water out, but these failed on a regular basis. A diver was sent to close a valve and he thought he had, but the flooding continued. The valve had been fitted with a left hand thread, not a right hand one as believed and he had opened a closed one in error.

In January 1881, a snowstorm severed communications and the supply of fuel for the pumps and consequently, timbers for the tunnel were used as fuel for the pumps, further delaying tunnelling.

Shortly after this this incident, salt water burst into the tunnel on the English side and bargeloads of clay had to be brought in to seal the leak in the river bed. And in 1882, the Great Spring struck again and then in 1883, a very high tide brought sea flooding when the sea overtopped the sea wall and again flooded the tunnel.

By August 1884, the tunnel had been completed and on 17 October 1884 the two headings met and the brickwork lining completed in April 1885. The 27 inch thick tunnel lining needed 77 million bricks and 37,000 tons of Portland cement.

But the Great Spring was not finished as pressure behind the bricks increased to 57 PSI causing leaks and distorted brickwork. This brought he engineers to decide that they had to install pumps in the tunnel to pump out 20 million gallons of water a day and this carries on today.

The first train ran through the tunnel on 9 January 1886 and carried coal and passenger trains commenced on 1 December that year. The tunnel had cost just under £2million and took 15 miles off the original London to Cardiff route and 95 miles off the Exeter to Cardiff route.

In 1887, 7,776 trains used the tunnel carrying 241,778 wagons while 30 years later these numbers were 23,122 trains and 1,085,892 wagons respectively.

This is why so much soot has had to be removed from the tunnel.

External Links

Liverpool street station in the UK at rush hour

Help and advice with train travel in the UK

Train travel in the UK should be a pleasure, not a headache. So if you’re taking a train on the UK railway, start your journey here. We’ve lots of hints, tips and advice to help you find your way around, travel smoothly and arrive in style by train.

Read more

eurostars passing at 375mph from the drivers seat in france


Welcome to our #railchat page. Our #railchat discussions take place over on Twitter, with our resident expert Phil Marsh. If you would like to get involved, please do join us, and use the dedicated hashtag #railchat. See our previous discussions below:

Read more

Old steam train in Shanty Town Museum

Locomotives & Engines

Our rail network would be nothing without them. From Stephenson's Rocket to British Rail Class 92 and beyond, explore the facts, information and anecdotes behind the steam, diesel and electric locomotives that built the age of the train.

Read more


Latest Tweets


Edinburgh Castle

Edinburgh to London Kings Cross by Train

Book in advance with and save 85% advanced bookings.

From £21.00

Buy now

Millenium Dome, London

Birmingham New Street to London Euston by Train

Book in advance with and save 92% on advance bookings.

From £6

Buy now

You may also like...



Follow us on Twitter and become part of our rail community! is the easy way to buy cheap train tickets online.



Control your business fuel costs with a supermarket fuel card – free to apply, no ongoing account charges and no monthly minimum spend.