Why does the tunnel need to close for six weeks if it only takes three minutes to fit a bracket?

It may take a pair of skilled workers just three minutes to fit a metal bracket to the roof of the Severn Tunnel, in readiness for the future electricity feeder cable. But at 4.5 miles long, the tunnel needs 7,000 brackets for each of the two feeder cables, and then engineers will start to attach two continuous aluminium rails, with their own supporting brackets, to the tunnel roof.

A copper conductor cable will slot into a groove on the underside of each rail. The new InterCity Express trains will collect the 25kV power from the cable as they zip through the tunnel.

Two sections of conductor rail were installed in the tunnel last spring, for test purposes.

Video Loading

Can't they just shut one side of the track or work at night?

Attaching equipment to the tunnel roof can’t be done with one track closed and the other open to passing trains, and Network Rail is installing feeder-cable brackets by closing the tunnel on Saturday nights. However, this is not an efficient way of working because it takes time to close the railway and move equipment and workers into the tunnel. Time must also be allowed to clear and check the tunnel before it reopens a few hours later.

Round-the-clock access to the tunnel roof from September 12 will allow engineers to undertake the repetitive installation processes without interruptions.

Anthea Dolman-Gair, senior programme manager for Network Rail Wales, said: "During the six-week closure, Network Rail’s army in orange will be working day and night to install over eight miles of overhead conductor rail. The scale of the engineering challenge involved and the extensive amount of machinery required mean that the closure is unavoidable.

"Without a solid six-week closure, it would take engineers a minimum of five years of weekend working to complete the upgrade, causing significant long-term disruption for passengers and delaying electrification in South Wales.”

An electrification bracket is installed inside the Severn Tunnel

Will they be doing anything else while the tunnel is closed?

Civil engineers will take the opportunity to finish lowering the track inside the tunnel to enable larger freight containers to pass through, potentially unlocking new business opportunities for Welsh ports in handling containerised imports and exports.

Most of the Great Western line will be electrified normally, with the copper conductor cable suspended from masts alongside the tracks. Several Great Western tunnels, including one of the pair at Newport, need overhead conductor rails because there’s simply not enough space above the trains for the normal system.

Swiss specialist Furrer+Frey has supplied overhead conductor rails for more than 180 tunnels, but never in such old tunnels where trains pass at such high speeds. Special supporting equipment had to be designed because the track in the Great Western tunnels lies on a traditional bed of stone ballast and will move slightly over time, unlike the track clipped to concrete bases in modern tunnels.

The Severn Tunnel also has a unique environment. Its opening in 1886 was severely delayed after workers struck underground streams. On average, 14 million gallons of freshwater are pumped each day from the tunnel’s sump near Caldicot.

An exceptionally high grade of stainless steel has been used for Severn Tunnel brackets, to help the equipment last for 80 years where pollutants from diesel trains mix with humidity to create a corrosive atmosphere. Diesel trains will continue long after electrification, because most routes each side of the Severn will be electrified.