Network Rail is to start £10 million of essential repairs to the Blackheath Tunnel between Blackheath and Charlton from 01 June to 10 August inclusive.

Southeastern trains which normally run through the tunnel will be diverted via other routes during the closure.

The one-mile-long tunnel, which opened in 1849, is prone to leaks and damage from water which filters through the ground above into the tunnel. While the tunnel is safe, the wet conditions are damaging the track and electrical equipment, which has caused around 1,000 minutes of delay to Southeastern customers in the past year.

Over the course of the entire programme of works, Network Rail will replace tens of thousands of bricks damaged by the freezing and thawing of rainwater and install a new concrete tunnel lining to stop water leaking into the tunnel, which will be crucial for the reliable running of the railway long into the future.

They will also strengthen several filled-in construction shafts that were used during the tunnel’s construction, rebuild drainage systems and carry out other essential track work including ‘wet bed’ removal – waterlogged sections of track that cause bumpy train rides for customers. These extra works will mean less disruption for passengers in the long term.

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A closure of 10 consecutive weeks is needed because carrying out work in cramped and narrow tunnels is incredibly difficult. If traditional weekend working was used, engineers would spend 50% of the time available bringing materials and plant in and out of the tunnel, leaving limited time for actual repair work.

David Davidson, Network Rail’s Kent route director, said: "This will be the largest programme of repairs to the Victorian tunnel in its lifetime, and while we recognise it’s an incredibly long closure, 10 weeks is still the least disruptive option for our passengers.

"The alternative would have been an entire year of weekend closures. Delaying the work into the future is not an option either as the track and equipment in the tunnel would only get worse and cause even more delays and disruption to journeys.

"By carrying out the repairs over 10 consecutive weeks we can work more efficiently, give passengers time to plan ahead, and deliver the reliability benefits to customers sooner. We’ve also chosen the summer because data shows the railway is typically quieter, and with less rainfall and drier conditions inside the tunnel, it’s much safer for our colleagues doing the work, allowing quicker progress.

"We'll do as much work as possible this summer, however, we expect that further work will be needed next summer. During this closure, engineers will use a specialist industrial hoover with a huge wire brush to clean away the soot which has built up over nearly two centuries of use, so that we can inspect the brickwork more thoroughly and identify where repairs are needed.

"We're really grateful to passengers and are working closely with Southeastern to make sure everyone understands the changes and impact to their journeys, and if further work is needed, we’ll engage closely with stakeholders and passenger groups."