Extreme heat and dry weather – like the conditions we’ve had in many parts of Britain this summer – can cause real problems for your rail journeys.
Soil moisture deficit is one of them, with soil moisture at their lowest levels since our records began in 2009.
Lots of very hot and dry weather can dry out the clay soil that much of our railway is built on, causing it to shrink. This is because trees and vegetation try to suck up any water left in the ground.
When this happens, the track becomes uneven so we need to slow trains down to keep you safe. It isn’t safe to run trains at full speed if the track moves.
In some cases, we need to introduce a reduced timetable so we can give you more certainty about rail services – and trains can run more reliably. In other cases, we may need to close sections of track.
Please check your journeys with National Rail or your train operator if your journey is affected.
What it means for your journeys
Safety is always our priority. We can only start permanent repairs once the soil stabilises so it will likely be winter before we can begin.
Where we can, we restore a level ride by lifting the track and compacting the ballast – the stones that support the track. But this is only a short-term fix until the ground stabilises, sometimes lasting just a day or two.
Once moisture returns to the track in autumn, the tamping is much longer lasting and train services can return to running at line speed. We often do this work overnight when no services are running.
Where essential, we’ll remove trees to limit further moisture loss while taking care to minimise the environmental impact. And we’ll let our neighbours near the railway know what we’re doing and when, well before the work starts.
Thank you for bearing with us while we work hard to keep you moving safely.
Where does it typically happen?
It can happen across Britain but there are different soil types across the country. Soil moisture deficit is a particular problem in Southern England, where there’s lots of clay in the ground.
One of the areas most susceptible is the South West of England line between Salisbury and Exeter, where about 60 miles of railway are built on clay embankments and much of the route is single-track. During prolonged periods of hot, dry weather, these clay soils can shrink, causing the track to move slightly.
Because much of the route is single-track, there are only six locations where trains travelling in opposite directions can pass each other. This means even small speed restrictions can quickly affect other services along the route and have a greater impact on the timetable than they would on busier double-track routes.
South Western Railway and Network Rail Wessex continue to work closely together to monitor conditions, carry out targeted maintenance and prepare for periods of prolonged dry weather. Where timetable changes become necessary, we’ll keep you informed through the usual travel information channels.
The problem also occurs in East Anglia. Dried out clay in Essex can affect Greater Anglia, c2c and Elizabeth line services. And north of Ely in Cambridgeshire, the peat soil underneath the railway behaves in a similar way to clay after prolonged periods of hot and dry weather. This can impact services in the area, including on the Great Northern route to and from King’s Lynn.
The problem with clay
Clay can absorb a lot of water when it’s wet but can severely shrink when it’s dry. This means we see more track movement due to the significant ground shrinkage.
So why do tracks run on top of clay? The Victorians didn’t know much about the makeup of the soil when they built much of our railway in the 1800s. Engineers then didn’t know how the railway would react in the dry summers we’re experiencing, which they couldn’t have predicted back then.
How we‘re managing soil moisture deficit
We can’t prevent prolonged periods of dry weather – but there’s a range of measures that can help reduce their impact on the railway.
Since 2025, South Western Railway and Network Rail Wessex have continued to strengthen the West of England line by closely monitoring locations most susceptible to soil moisture deficit. Together, we’ve carried out extensive tamping work, installed more than 5,000 tonnes of new ballast and removed more than 200 dead and diseased trees from vulnerable embankments.
We’re also working with the University of Southampton to better understand soil moisture deficit and develop more long-term engineering solutions that will help make the railway more resilient to hotter, drier summers in the future.
In Anglia, before this summer season, teams dropped thousands of tonnes of ballast to help keep the track as smooth as possible. We expect to drop 49,000 tonnes 2026 in total. Extra late night engineering work is also taking place across the region, including the use of tamping machines. Last year, engineers in Anglia treated about 100km of route using tampers, helping to keep services running on time by improving the condition of the track.
Where conditions mean trains need to run more slowly, these decisions are always taken to keep you and our colleagues safe while helping to provide the most reliable service possible.
How we prepare
We usually know when to expect soil moisture deficit, but in extreme cases – like the ones we’re seeing now – we need to deal with it earlier and for longer.
The best defence against soil moisture deficit is preparation. We monitor moisture levels and track movement, and enhance maintenance where it may cause concern. We may need to remove trees and vegetation that are absorbing water and drying out embankments. This is the most appropriate strategy because re-engineering all our clay embankments would cost tens of billions of pounds and take decades to complete.
We’re starting to plan for the effects of soil moisture deficit at the same time as our hot weather preparation. We ensure the track is at the best possible quality before the hot weather sets in by carrying out additional maintenance in the winter and spring.
