How Railway Track Is Designed and Maintained to Cope With Summer Heat
- May 9
- 9 min read
As rail temperatures rise, so do the forces acting within the track. The question is not just how hot the weather gets, but whether the track system is built, maintained and managed in a way that can cope with it. In this article, I break down how that works in practice and why it matters.
The summer months are nearly here. The days are longer, and temperatures are starting to rise.
As temperatures rise, rails warm and forces start to build.
As the weather gets better, people want to get outside and enjoy it. Rails, on the other hand, want to expand. Metals, such as the steel used to make rails, expand as they are heated. When installed, rails may either be joined mechanically or welded into long continuous lengths, depending on how that section of track is built. They are held in place and restrained by the fastening system. After all, we do not want them moving under a passing train. So they want to expand, but cannot. And the hotter they get, the more they want to expand.
Just by how much might surprise you. Over a 20°C temperature rise, a 1 kilometre length of rail would try to grow by around 230 mm. Over a 30°C rise, it would be closer to 345 mm.
When this expansion is effectively restrained, compressive force starts to build in the rails, almost like loading a spring. But when this compressive force becomes too much, the track buckles, releasing that force through sideways movement.
So how does the railway keep this compressive force in check and avoid buckles?
It is kept in check through the design of the track system, correct maintenance, and mitigations where they are needed. What does that look like in practice? Let’s find out.
Jointed and welded track solve the summer problem differently
Nearly all sections of track are exposed to hot weather, and the thermal forces that increasing rail temperatures bring. The way that the track responds, however, depends to an extent on how it has been built.
The biggest difference, however, is how the rails are joined together, because that has the greatest influence on how the thermal forces are managed. Rails are primarily joined in two main ways:
Welded together to form continuously welded rail (CWR)
Joined using fishplated joints to form jointed track
There are variations within both, especially in the types of joints used, but these are the two main forms that shape how hot weather forces are managed.
These two forms of track deal with rail expansion, and the thermal forces that come with it, in very different ways.
In continuously welded rail, the rails are fully restrained by the track system. To manage those compressive forces, the rails are installed in a state of tension. This is done through a process known as stressing, where special equipment is used to stretch the rail to the length it would be at the target stress free temperature before it is finally welded. When the rail reaches the Stress Free Temperature (SFT), it is neither in tension nor compression. Once the rail temperature rises above the stress free temperature, compressive forces start to build. The difference is that stressing means this happens from a much higher point than if the rail had not been stressed at all.
Jointed track takes a different approach. The regularly placed joints in the rail are left with gaps between the rail ends. They are called expansion gaps for a reason. They are left to allow the rails to expand without becoming tight up against the next rail and starting to compress each other. These gaps are set to specific widths, depending on the rail temperature at the time. The temperature at which those gaps fully close is known as the Joint Closure Temperature (JCT).
Both track types rely on maintenance if they are to manage thermal forces as intended. For CWR, that means restoring the correct stress condition whenever rails are changed, and where that cannot be done immediately, restressing them as soon as possible. For jointed track, the joints need to be kept in good condition, including lubrication where required, so they can function as intended. The joint gaps also need to be inspected regularly, and adjusted if they are not at the correct size.
It’s Not Just The Rail
The track is made up of multiple components, and they all have a part to play. The management of thermal forces isn’t just down to the rail itself.
Let’s move down through the track system and look at each component.
Fastenings and pads - The fastenings secure the rail to the sleeper, whilst also helping to restrain it longitudinally by exerting load on the toe of the rail. Helpfully, this is known as toe load. The pads sit between the rail and baseplate or sleeper, and also play a part in how load, vibration and movement are managed.
Sleepers - When it comes to managing thermal forces, sleepers do two main things. Firstly, they provide a surface, the sleeper sides, for the ballast to sit against. Secondly, they contribute weight to the track system. The higher the weight of the track system, the more force required to move it. This is where concrete sleepers have an advantage over other materials such as wood, given their higher density.
Ballast - The biggest component in the track system, ballast is key to resisting track movement. Packed, and importantly consolidated, around the sleepers, it resists any movement through its mass and interlocking stones. A key factor, however, is the amount of ballast and its condition. The areas around and between sleepers should be full, covering the sides of the sleeper. At the sleeper ends, the ballast shoulders should be correctly profiled, giving additional restraint against sideways movement. Because ballast is made up of angular stones, it interlocks as it consolidates, making it stronger. However, ballast disturbance does temporarily weaken it (more on this later). Ballast that is fouled also loses the ability to interlock properly, again weakening it.
When it comes to managing the thermal force that the rails exert on the track, all the components have a part to play, but ballast has the greatest influence on how much movement the track can resist.
Component selection and track construction type are decisions taken when a section of track is first installed, and they do a lot to determine how that track will perform when the weather warms up. Maintaining the track so it continues to perform is just as important, but maintenance work is not without its own risks.
Maintenance is Necessary, But at the Right Time
Everything needs maintenance if it is to keep functioning as designed. The railway is inspected to find faults and defects, and work is then planned to rectify or remove them. But the work done to remove a risk can itself introduce one, and during hot weather that is especially true.
Above, I spoke about how disturbing ballast can temporarily weaken it by reducing its consolidation. Most work to rectify track geometry faults, change sleepers, or carry out ballast renewal involves disturbing the ballast. Reducing the strength of the ballast to resist those forces, at the same time the rails may already be exerting high levels of force, needs to be carefully considered and managed. If the compressive force in the rail is already high enough, even a local reduction in ballast strength can be enough to allow the track to buckle.
The only way for the track to fully regain its strength is through reconsolidation, as the ballast becomes packed and interlocked again. Mechanical means such as wackers and Dynamic Track Stabilisation can help to a certain extent, but most reconsolidation still comes through the passage of trains over the area. How long that takes depends on the amount of train traffic over the area. That period between disturbance and reconsolidation is when the risk of a buckle is higher, so mitigations must be taken. I will come back to that in the next section.
The other type of maintenance that becomes more risky in hot weather is any work that involves the rail itself. Cutting the rail or undoing a joint can suddenly give it the freedom to expand. That can then make reinstating the rail correctly extremely challenging, and if it is not properly managed the rail can be left carrying the wrong level of stress.
Maintenance is necessary to remove risks to trains, but in hot weather how and when it is carried out needs to be carefully considered. If it must be done, then the correct processes and mitigations need to be followed to prevent a buckle from occurring.
Critical Rail Temperature, The Way The Risk Is Managed
I have covered the risks that hot weather brings, how the track is designed to deal with them, and how maintenance can introduce further risk. I also touched on the need to put mitigations in place to prevent buckles where work has been undertaken.
So, on a day-to-day basis through the summer, how is all of this managed? How do engineers and railway operators keep on top of the risks whilst keeping trains and passengers moving?
This is where the Critical Rail Temperature (CRT) comes in. It is the rail temperature above which the risk of a buckle is considered high enough that mitigations need to be put in place.
How is it determined? In the UK, the CRT for a section of track is based on a number of inherent factors, including:
The SFT or JCT of the rails
The type of track construction, such as sleeper type, fastening type and sleeper spacing
Track curvature
Other factors are also taken into account, such as:
The level and condition of the ballast
Whether the track has been disturbed, and the extent of that disturbance
From there, different rail temperature thresholds are set for different mitigations. In the UK, these mitigations can include putting watchmen on site to look for signs of movement, followed by speed restrictions of varying severity.
The CRT at a site can be raised by:
Raising the SFT or JCT where it is below the level stated in the standard
Topping up areas of low ballast or renewing fouled ballast
Giving disturbed ballast the time it needs to reconsolidate
Manage the Speed, Manage the Risk
You may have stood on a platform on a hot summer day and heard that your train is delayed due to heat-related speed restrictions.
Why is reducing train speed the primary mitigation against buckles when rail temperatures start to peak?
Again, it all comes down to forces. When a train passes over a section of track, it exerts forces into the track. The faster the train is travelling, the greater the forces and dynamic effects it can impose on the track.
During hot weather, the track system is already dealing with high levels of force from the rails. The ballast and the rest of the track system are already restraining the rails and holding that movement in check. The additional loading from a passing train can be enough to overcome that restraint and trigger a buckle.
So, train speeds are reduced to reduce the additional loading from passing trains, and therefore the risk of that loading triggering a buckle.
There are Always Exceptions
Some areas require rail movement to be managed differently, and that is where adjustment switches or breathers come in. These are an assembly made up of two blades that allows controlled rail movement, whilst still maintaining a running surface for the train wheels. They are used at locations where rail movement needs to be accommodated in a controlled way, rather than simply restrained.
So where would you find these on track?
At interfaces between jointed track and CWR
At certain bridge interfaces, where the track may be fixed directly to the structure rather than supported in ballast, so rail movement has to be managed differently
At some tunnel mouths, where the difference between the thermal conditions inside and outside the tunnel can be significant over a short distance
Around certain unstressed switch and crossing layouts, where rail movement cannot simply be restrained in the same way as plain line CWR
Adjustment switches can offer good protection in the right locations, but they are also another asset that has to be inspected and maintained.
Summary
Hot weather on the railway is not just about high temperatures. It is about how the track deals with the forces those temperatures create.
Whether the track is jointed or continuously welded, the same principle applies. The rails will try to expand. The question is whether the track system is built, maintained and managed in a way that can handle it.
That comes back to the whole system. The stress condition of the rail. The condition and quantity of the ballast. The restraint provided by sleepers, fastenings and pads. The effect of recent maintenance. And where needed, the use of adjustment switches or other movement management arrangements.
When that margin starts to reduce, the railway has to respond. That is where CRT, site monitoring and speed restrictions come in. Not because speed restrictions remove the thermal force, but because they reduce the additional loading that can trigger movement.
In the end, summer track safety is not about one component or one rule. It is about whether the whole system is still doing its job.
If you want a clearer understanding of the parts of the track system and what each one does, my Track Component ID Guide is a good place to start.
Want to find out more about how the railway deals with the ever changing challenges of the weather, I have put all my videos that might be of interesting into a playlist.
Check it out HERE
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