Britain's largest tunnelling machine is being used on a £1bn electrification project to improve rail services across north-west England.

'Fillie', the tunnel boring machine (TBM), has begun work to reconstruct Farnworth Tunnel as part of an electrified railway project between Preston and Manchester.

At 9m in diameter at its cutter head, 'Fillie' is bigger than the machines used to build London's Crossrail (7.1m), the Thames Tideway 'super sewer' (8m) and even the Channel Tunnel (8.8m).

Upgrading the railway through Farnworth Tunnel posed a particular engineering challenge, as the twin tunnels which carry the railway under the main A666 Manchester to Bolton road aren't big enough to accommodate the overheard lines which will power a new fleet of electric trains.

The solution to this is to keep one of the 270m-long tunnels open to trains, while the other – just 1.5m away – is pumped full of 7,500m3 of foam concrete before a completely new tunnel, big enough for two tracks and their overhead power lines, is re-bored through the new concrete by the TBM.

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Nick Spall, from Network Rail, said: "The work we're doing at Farnworth is part of a major upgrade to electrify one of the region's busiest commuter routes. It will mean faster trains with more seats, giving passengers better journeys and boosting the economy.

"This is a once-in-a-lifetime opportunity to work on a project that is technically challenging but will also deliver big benefits for local people."

Transport Secretary Patrick McLoughlin MP visited the site on 11 August to see how the tunnel is being constructed.

He said: "This one nation government is determined to close the economic gap between north and south. Investing in transport – something that successive governments have failed to do sufficiently – is vital to making that happen.

"We had a choice - building the infrastructure our country needs, or letting our transport system become a brake on growth and opportunity. We have chosen to invest for the future.

"Across the north, that investment is already having a huge impact, with programmes – like the impressive work at Farnworth – underway to upgrade our railways and our roads. We are determined to keep the momentum going."

Around 120 people are working 'round-the-clock' on the project, which has already seen:

• The complete demolition of Farnworth and Moses Gate train stations, with work started to rebuild them on their new alignments.

• Over 1,600m of tracks lowered.

• 7,500m3 of foam concrete pumped into the tunnel.

• The construction of a 300m motorway standard access road, removing 40-120 lorries from residential roads each day.

• A new footbridge at Kearsley.

• The installation of a concrete lining and permanent rib supports to the operational tunnel.

The team remains on course to reopen the tunnel on 5 October.

(LM)