Trains: How the Severn Tunnel connects beer, diving and oasis

The leaking water is in your beer, it’s the place where an Oasis classic was born and where one of the first dives took place.

It is also one of the most dangerous places to build a railway.

More than 20 Olympic swimming pools worth of water is pumped out of the Severn Tunnel every day. Some of that water is used in drinks such as Budweiser and Stella Artois, which are brewed a few miles away.

You can imagine that you don’t realise how difficult it is to make your train journey happen when travelling between Wales and England, over this engineering marvel deep beneath the UK’s longest river.

It was once the longest underwater tunnel in the world. It took 13 years to build the tunnel by hand, using picks, shovels, drills and dynamite. That was almost 140 years ago.

It is a place where it always seems to be drizzling. And it is the constant seepage of salt water from the river above that makes it one of the most inhospitable areas for a railway.

When it’s dark outside for three minutes and 40 seconds as you drive 75 mph through the tunnel on one of the UK’s busiest rail lines, linking London and Bristol with South Wales, most people are completely unaware of the work that has gone into keeping them moving.

It is the lowest point on the UK’s 35,000 kilometre rail network. The track is replaced more frequently here than elsewhere due to the highly corrosive, salty atmosphere at almost 50 metres below sea level.

That’s why the Severn Tunnel is currently closed for the longest time – except when it was electrified a few years ago – since it opened in 1886.

Normally, around 200 trains pass the seven-kilometre route every day.

This is not yet the case. Engineers have closed the route for 16 days, until 19 July, to renew the underwater line towards South Wales. This involves swapping all 66 rails, 10,800 sleepers and the 22,000 tonnes of ballast supporting the track.

“The ingress of water, diesel fumes from the trains and the aggressive nature of the minerals in the tunnel are all damaging the track, which is why we now have to renew the track every five years,” said Brian Paynter of Network Rail.

“The sleepers and ballast are replaced every 15 years. In the UK this is probably the quickest way to renew a railway line because of the hostile environment.”

Outside the Severn Tunnel, the tracks are typically replaced every 25 to 30 years.

Despite 76.4 million stones – often 10 layers thick, surrounded by 1 metre of rock – holding back the tide from one of the world’s most notorious bodies of water, the Severn still seeps through the tunnel wall.

Keeping the tunnel, which runs beneath a fast-flowing river with the third highest tidal range in the world, dry has been a challenge since digging began beneath the Severn Estuary in 1873.

It was only the second underwater tunnel attempted, but it was more than 13 times longer than the first, the 400-metre-long Thames Tunnel in London, which opened in 1843.

In Victorian times there were no laser-guided radar equipment or tunnel boring machines to build this feat of engineering.

It was blood, sweat and tears, with a lot of geology and math and some trial and error.

“Engineers were very different back then,” Mr Paynter adds.

“I can imagine how difficult it was. Without computers, telephones, lasers and sonar, they did a fantastic job. And 150 years later, it is still a success.”

The tunnel and feeder routes are approximately 7 miles long, allowing the line to go almost 450 feet (137 meters) below the lowest point of the Severn. Just make sure the gradient is gradual enough for fast trains to race through.

The work was slow and laborious, with pumps pumping away the water. But disaster struck six years into the construction, when excavators stumbled upon an underwater well.

Workers were less than 119 metres (400 ft) from connecting the two tubes when the tunnel first flooded in 1879.

Construction was delayed for more than a year and could only resume after Alexander Lambert completed one of the first dives in history, using Henry Fleuss’s newly developed self-contained rebreather apparatus, which allowed him to close a watertight door.

The Great Western Railway’s chief engineer, Sir John Hawkshaw, also ordered the well to be sealed and the tunnel path lowered to prevent further problems.

Water entered the tunnel on a few more occasions, notably in 1883 and 1885, so engineers installed larger, permanent pumps on the Welsh side.

And yet those enormous pumps still work 24/7 to pump 75 million litres of water out of the tunnel every day, some of which ends up in your glass of beer.

The tunnel cut the journey time between Bristol and South Wales by around an hour – and was the quickest way to cross the estuary before the first Severn road bridge opened in 1966.

Until 1987 it was the longest underwater tunnel in the world and for over 100 years it was the longest railway tunnel in the UK.

But keeping it operational and limiting delays is a mammoth task that requires constant attention. Furthermore, the planning of each work programme takes five years and starts as soon as the previous cycle has ended.

Still, a tunnel delay in the 1990s gave Noel Gallagher time to write one of Oasis’ most popular songs and a Britpop anthem of the 1990s.

A fan favourite and a fixture on Oasis’ setlist for 15 years, Acquiesce found their musical origins beneath the sea, between England and Wales.

“I wrote that song on the train on the way to Loco Studios in South Wales for the Definitely Maybe sessions,” Oasis songwriter Noel Gallagher once said.

“I was on my own. I was stuck in the Severn Tunnel and I had my acoustic guitar. I thought, ‘Give me the guitar, what does that word Acquiesce mean?'”

Last year the tunnel was closed for 10 days to replace the track. This year it will be closed for 16 days for similar work to take place 24 hours a day. The work is estimated to cost around £10 million.

Water drains are installed in the top of the tunnel to prevent salt water from dripping onto the tracks.

Fans have also been installed to create a constant breeze, thus reducing the strain on the steel rails and electrical wires.

Each of the 66 new railway lines being built in July will be larger than two football pitches (215 metres long) and will cost around £14,000 each.

Some 10,800 concrete sleepers are also being replaced, costing £90 each.

“There are sceptics who say you are blocking the railway line again,” added Network Rail’s Mr Paynter.

“But I do it for the greater good of ensuring a resilient and safer network. This is a temporary period of disruption, but if we don’t replace the track, it’s all over. It’s short-term pain for long-term gain.”