We go inside the Severn Tunnel as it closes for major renewal work

Travelling through the Severn Tunnel is something many of us are used to.

But today we're experiencing it in a very different way - journeying on foot to see construction work up close.

The tunnel is closed from July 3 to July 19 so engineers can replace more than 7km of track.

It's a mammoth task, with teams also installing more than 10,000 sleepers and over 22,000 tonnes of ballast.

Trains from South Wales to the West of England and beyond are rerouted via Gloucester while the tunnel is shut.

Nick Millington, Network Rail's Route Director for Wales & Borders, tells me about the complexity of managing the tunnel.

"It's an incredible feat of engineering. It's 7000 meters long and for many years was the longest tunnel in Great Britain. It does take care and attention and we've got a number of different things going on in this tunnel at any one time."

"We've got a 25,000 volt power distribution system overhead that powers the trains. We've got the track railway system below, drainage, and we've got signalling equipment and the structure of the tunnel itself.

"It does take a lot of inspection and a lot of care to keep the railway safe and operational down here."

Powerful pumps keep the tunnel from flooding.

As we reach the deepest point of the tunnel, Programme Director Ross Mahoney explains the exceptional challenges the team have to deal with.

"There are particular conditions that this railway is exposed to that we don't see anywhere else on the network.

"We see higher levels of rail corrosion due to the salt content of the air. Despite this tunnel lining being ten bricks deep in places we still see sea water seepage into here."

The result is that track that would normally have a life of 25 years has to be replaced every ten instead.

"We appreciate we've got the railway blocked... is an inconvenience for passengers for the 16 day period" he says.

"But it is fundamental to the way in which we manage the infrastructure down here.

"By the time we're finished, we'll see the entirety of this line through the tunnel renewed from end to end."

The tunnel is a vital link between South Wales and England.

Keeping it operational requires considerable effort, resource and planning.

But its continued survival is a testament to the skill of the Victorian engineers who constructed it, and the teams tasked with supporting it now.

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