Last updated: Thursday 9 July 2026
What is Soil Moisture Deficit (SMD)?
Parts of our railway are built on embankments containing clay soil, which normally contains a lot of water.
When we experience high temperatures and dry conditions, typically in the summer, water is removed from the clay soil through heat radiation and nearby vegetation. This causes the clay soil to shrink, which disturbs the ground our tracks are built on.
This is known as Soil Moisture Deficit, or SMD, and it can have a big impact on our train service.
How does it affect the railway?
When railway embankments shift it affects track geometry - the precise positioning, alignment and measurements of our tracks. Sleepers can move out of position, shifting sideways or downwards, which affects the rails on top of them.
Tracks need to be level for trains to run at full speeds, and if the levels, or ‘track geometry’, isn’t right, temporary speeds restrictions are put in place to ensure they run safely.
The West of England Line, an important part of our network, is particularly vulnerable to SMD. The route serves a total of 21 stations over 120 miles and connects London with Exeter St Davids in Devon, Salisbury in Wiltshire and Yeovil Junction in Somerset.
The line between Salisbury and Exeter is a predominantly single-track railway. Speed restrictions along long stretches of single-track can cause cumulative delays and disruption along the whole line, which can have a knock-on impact on other services across the network.
In these instances, temporary timetable amendments can offer better reliability for customers on the West of England line.
Unlike buckled rails or issues with signalling equipment, there is no quick fix for track affected by SMD. It can take weeks or even months of rain for moisture levels to return to normal.
How do we prepare for SMD?
While we cannot prevent SMD, we are better prepared for it, thanks to continuous monitoring of soil moisture levels, enabling earlier identification of risks.
Since we removed the West of England Line’s temporary timetable in November 2025, we’ve worked hard to restore track conditions to a high standard. Over the last year, we have:
We’re closely monitoring soil moisture levels and weather patterns, so that our engineers can understand when track conditions might be affected and react quickly. If we do need to act, this will mean we’ll be able to provide you with more notice about service changes and reduce disruption.
How do we fix the track?
We’re checking track conditions through regular inspections and by using technology such as the New Measurement Train (seen above, passing through Yeovil) which tells us where track geometry needs to be corrected.
To restore track geometry to normal we use tamping machines, which precisely moves ballast back to where it’s needed. However, we can only do that when the clay soil regains moisture, otherwise the tamping will soon be undone by the ground continuing to move.
When the embankments have regained their moisture through rainfall, we’ll tamp the track and remove any speed restrictions. If we experience prolonged spells of dry weather, like we saw in 2022 and 2025, it can take weeks and even months for moisture levels to return to normal.
Longer term improvements
We’re exploring ways to minimise the impact of SMD on both our track and services for customers.
Our engineers have been working with experts at the University of Southampton to understand how better to manage track during SMD conditions, while our train planning teams have looked at a range of options to keep our services as reliable as possible.
Long-term initiatives being explored include:
- Bio-cementation – injecting nutrients into the layer of soil below the track to create a calcium carbon skeleton, which provides more support when the clay soil shrinks
- Injection grouting – injecting expanding polymer foam to fill any voids left by shrinking soil, strengthening the formation of the track
- Self-adjusting sleepers – using sleepers that have a built-in reservoir of chippings, so that when the clay soil below shrinks and leaves voids, the chippings fill the void and prevent the sleepers from moving
- Micro-piling – installing piles in the gap between the rails to provide extra support to the sleepers, which will stiffen the formation of the track and reduce its movement when soil shrinks
Latest information
Since December 2025 we have closely monitored soil moisture levels across our network and are regularly inspecting more than 30 sites between Salisbury and Exeter that are at risk of SMD.
Following a period of extremely hot and dry conditions, which included heatwaves and the highest temperatures ever recorded in June, soil moisture levels are decreasing, and track conditions are changing.
To keep trains running safely, and to operate a reliable service between London Waterloo and Exeter St Davids, an amended timetable may be needed in the coming months.
We will provide as much notice as possible of any timetable changes. Please check back for updates.
