Train passengers are being advised the railway between Whitehaven and Corkicle is closed after recent surveys.
Network Rail has been working inside Bransty Tunnel for the last five days, carrying out assessments ahead of a full track and drainage renewal next month and September.
It involved doing tests to assess any weight limits needed for the heavy plant and machinery needed for the planned renewals.
However, Network Rail said the tests uncovered some areas under the railway which would need strengthening before any work can progress.
Specialist engineering teams are now planning exactly how to carry out the extra ground strengthening work.
After assessing the initial survey data by specialist expert engineers, Network Rail has closed Bransty Tunnel to all trains until further notice.
Rail replacement buses are in operation between Workington and Sellafield
Darren Miller, Network Rail’s North West route infrastructure director, said: “We’re sorry to passengers impacted by this unexpected longer closure of the railway between Whitehaven and Corkicle.
“The preparatory surveys have revealed previously unknown ground conditions which we need to investigate further so we can carry out repairs and get the line reopened as soon as possible.
“We’re working closely with train operator Northern and our freight customers on alternative routes and rail replacement bus services. I’d encourage anyone planning to travel on the Cumbrian Coast Line to regularly check National Rail Enquiries while we work hard to restore services through Bransty Tunnel.”
Craig Harrop, regional director for Northern said: “We understand that this news is disappointing for those who rely on the Cumbrian Coast Line. We will continue to do everything we can to keep people moving along the route whilst the tunnel remains closed.
“I’d like to thank our customers for bearing with us while this essential work takes place, and encourage everybody to check before they travel as journeys will take longer than usual.”
It’s not yet known what has caused the conditions underground, but in recent years the tracks inside the tunnel has been affected by floodwater containing iron ochre, thought to have penetrated the tunnel structure from historic mines in the area.
Network Rail is working with the Mining Remediation Authority to determine if these latest surveys have discovered any previously unmapped mine workings beneath Bransty Tunnel.
The most recent survey data is now being processed so plans for the extra work needed to strengthen the tunnel floor can be developed swiftly, Network Rail said.
This must be done before the track renewal can take place, and it is likely that will happen later than expected, Network Rail said.
The exact timeframes for how long the repairs and subsequent track improvements were still being determined.
