What is 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 the South West via Exeter St Davids in Devon via Salisbury in Wiltshire and Yeovil 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, and 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 soil moisture deficit. 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:

• Closely monitored more than 30 sites that are at risk of SMD
• Installed 5,500 tonnes of new ballast – the stones beneath the track that provide support
• Removed almost 250 dead, diseased or dying trees from the line
• Carried out an intensive programme of checks on track conditions through cab rides, geometry reviews and regular use of the New Measurement Train
• Worked with the Met Office to monitor weather patterns - the recent spring was the warmest spring on record in England and Wales, with just a fraction of the expected rain and very high temperatures at the end of May
• Commissioned research with the University of Southampton to improve understanding of SMD challenges and interventions

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 can react quicker we can provide you with more notice when we have to make service changes, and we can reduce the disruption that results sometimes.

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 – using nutrient solutions to accelerate naturally occurring bacteria, forming a calcium carbonate skeleton that strengthens the soil layer
• Injection grouting – injecting expanding polymer foam into the earthwork at various depths, filling voids and strengthening the soil response to SMD
• Self-adjusting sleepers – using chipping reservoirs within sleepers to fill voids created by shrinking clay soils, reducing settlement
• Micro-piling – installing piles in the four-foot between sleepers to stiffen the formation response to SMD