As Brunel’s Great Western Railway marks 175 years, DAVID CLENSY ventures down into the labyrinth of shafts, culverts and pumping drains that allow more than four million people to travel safely through the Severn Tunnel each year. Pictures by DAN REGAN.

ISAMBARD Kingdom Brunel, the great brain behind the Great Western Railway, didn't survive long enough to develop the idea of a railway tunnel beneath the Severn Estuary.

His original plans for the Great Western Railway ended at Bristol – with travellers needing to get off the train at Pilning and step aboard a ferry to Black Rocks Station if they wanted to climb aboard another train to take them deep into south Wales.

Brunel's focus was on connecting London with Bristol's docks – and therefore slashing journey times from the capital to New York.

Crouching room only: One of the many tunnels at the Sudbrook pumping station in Wales

It's a long way up: The spectacular view up one of the shafts at the pumping station

On August 31, railway enthusiasts will celebrate 175 years since Parliament passed the necessary bills to allow work to start on the Great Western line – an epic engineering challenge that would reshape the face of British transport.

But it was the Severn Tunnel, which was constructed 40 years later, that gave us the Great Western line we know and love today – 15 per cent of journeys on the line now make use of the tunnel, which equates to 4.7 million people each year who race beneath the estuary's waves at 75mph without giving it a second glance.

But we shouldn't overlook the mammoth engineering challenge presented by this extraordinary shortcut to Wales.

Even today, a network of enormous pumps run constantly to take the water out of the tunnel.

Almost none of this is seepage from the Severn itself. Most of the water drains down the gentle gradient of the tunnel as rainwater run-off from the land, but a considerable amount is also rising from an underwater spring that runs beneath the tunnel itself.

It's a sobering thought, that even today, if the pumps switched off or completely lost power, it would take 26 minutes for the entire Severn Tunnel to fill with water.

This vital work is carried out by a team of 18 men, working in shifts at the Network Rail managed Sudbrook Pumping Station, which stands beside the old Severn Bridge on the Welsh side of the estuary.

Shift maintenance official Dave Fuller and depot manager Andrew Williams agree to show me around the complex of underground culverts, sub-tunnels and stomach-churning shafts that are necessary to maintain the Severn Tunnel.

Inside the Sudbrook Pumping Station, Andrew begins by unrolling an ornate cross-section drawing that was used by the engineers who built the tunnel between 1873 and 1886.

"Everyone was amazed when they built the Channel Tunnel, starting from either side, and managing to meet in the middle," Andrew says. "But this was built in exactly the same way, more than a century earlier – only this was before they had modern drilling moles and global positioning satellite systems to help them."

The tunnel is about 50ft beneath the seabed, and 180ft beneath sea level.

"They did a tremendous job," adds Dave, who has worked at the pumping station for more than 25 years.

"First I'll show you the control room," he says, leading the way into a central desk area, surrounded by monitors, switches and gauges of every conceivable shape and size.

"This is where we control the pumps that are currently pumping out 10 million gallons of water a day from the tunnel," he explains, checking a computer screen to ensure he is giving the correct figure.

"But it's also where we control the enormous fans that are used to pump air down into the tunnel – which makes it a safe place to work.

"The double-tracked tunnel is about 4.5 miles in length with a downward gradient of 1:100 from the Gloucestershire end, with a similar gradient of 1:90 from the Monmouthshire end.

"Because of these gradients, the considerable quantity of water that is always to be found in the tunnel is culverted and gravitates to a common point at the lower end of both declines. This is removed by our high-capacity fixed pumping plant at Sudbrook.

"Additional pumps, remotely controlled from Sudbrook, are at the Sea Wall Shaft and at 1.5 miles west of the mid-tunnel, but most of the pumping is done here.

"The total installed pumping capacity exceeds 35,000 gallons of water a minute, though the average is 15,000 gallons per minute.

"All this control room is technically computer-run these days," Dave adds. "But nobody is brave enough to suggest removing the human being from the control seat.

"If the pumping stopped, the consequences are unthinkable. That's why we also maintain four diesel generators that automatically kick in in the event of a power cut, and each of these generators has the capacity to run all the necessary pumping equipment on its own.

"But when the power does go off, it can seem like a very long 30 seconds before the generator kicks in."

Dave leads the way to the 180ft lift shaft down to the level of the railway tunnel.

The fast, modern lift deceives the brain, and you hardly feel you've travelled anywhere when you step out into a tubular side tunnel that could be straight out of a James Bond villain's lair, and walk towards the whirring sound of the fans that is emanating ominously from the main tunnel.

But first we need to step inside an air-lock chamber, which protects the lift shaft from the awesome changes in air pressure created by a high speed train being thrust into the tunnel at 75mph.

Dave warns us that beyond the next door, we are straight on to the railway lines, and, right on cue, as he opens the door a Sprinter train goes hurtling past our faces.

"Wait until you see one of the high speed trains," Dave laughs, as he observes the startled expression on my face.

We wait a moment, but the only sound is the deafening roar of the fans, so we go to retreat back into the air-lock chamber.

But as Dave is about to shut the door, a curious hiss seems to gather around the gap between the frame and the door – as if the air is being sucked out from around us. In fact, it is.

"That's a train," Dave quickly explains, before reopening the door.

Moments later, and the windows of an Intercity 125 are flickering past the door at a thundering rate that's enough to momentarily take your breath away.

Back in the lift, alarmingly, Dave presses a button and we start to descend even further into the bedrock.

"Most people don't realise there is a second tunnel running parallel, but beneath the railway tunnel," he says. "We call it the Glory Hole, and it's designed so that all the air pressure running ahead of the speeding train has somewhere to go. It literally loops the air beneath the train and back out in the direction it came."

After a stooping shuffle along a long, narrow, dank tunnel, we step out into the cavernous Glory Hole.

As another train whistles past above our heads, the wind flurries around us, momentarily making the narrow opening from which we have just emerged, invisibly but powerfully impassable.

Back on the surface, Dave leads the way across the pumping station to another shaft, and we descend once again – this time in a rickety and slow lift, with windows, which allows you to get a true taste of just how deep we're descending.

"This one is not designed for the emergency services," Dave explains. "It is designed for engineers to check the pipes and equipment that run down the shaft – that's why it runs so slowly."

It's a sort of unsettling cross between Alice in Wonderland and Fred Dibnah, and I find myself strangely relieved to reach the bottom.

Stepping out at the base, it's difficult not to look up at the towering chasm that is disappearing into the darkness above our heads. But the sound of manically rushing water soon diverts your eyes downwards. Below us, is an enormous angry cauldron of white water.

"This is where we divert the underground spring, in order to then pump the water up to the surface," Andrew tells me.

"Then much of it gets diverted out to the nearby Interbrew Brewery. So a considerable amount of this water ends up in pints of Stella and bottles of Budweiser."

After another bent-double shuffle down a slimy brick-arched tunnel, we reach another flowing culvert.

"This water is just the run-off rainwater," Andrew says. "So it is always kept separate from the spring water, and this just gets pumped up and back out into the Severn.

"It really is a fascinating place. I've worked here for almost a decade, and you never get bored by the engineering. It really is very clever..."