A four-mile long speed restriction has been put in place on the West of England main line between Gillingham and Tisbury.

The railway is feeling the impacts of the driest July since 1936 as sections built on clay-based soil have dried out so badly that they have shrunk, leaving the track on top uneven and dangerous to run trains at full speed.

The restriction is likely to remain in place until October as engineers must wait for the dried-out soil to regain some of its moisture before a repair can be carried out.

Colum Cavanagh, head of track engineering for southern region at Network Rail, said: "We call this problem Soil Moisture Deficit and although we encounter issues with it most summers, this year has been absolutely unprecedented.

“We came into the hot season with our track in the best condition it’s ever been in, and yet now – late August – the soils around Gillingham have dried so badly the track is only able to take trains running at 40mph, down from 80mph."

'£15bn to £20bn' to rebuild

If Network Rail were to rebuild all 6,000 clay embankments with modern materials, it could cost anywhere between "£15bn to £20bn and take decades", according to Mr Cavanagh.

He added: “Normally we would take a piece of kit called a tamper in and sort the track out, a bit like shaking a duvet and getting it flat again. This year the problem is so bad that the soil is still shrinking and it’s going to be some weeks until it has stabilised enough for us to do the work to bring line speeds back up to normal."

Train services have had to be amended as the line is single – so no trains can pass each other – and trains are taking double the normal length of time to run between Gillingham and Tisbury.

Claire Mann, South Western Railway's managing director, said: "After two weeks of delays and short-notice changes to our services, this decision to introduce a revised timetable will allow us to run a resilient service and at least provide certainty to our customers in the West of England."

The full revised timetable is currently being finalised and will be published later this week.

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