Rail 101 - Why rails fail more often in winter

Rail failures are usually seasonal - Lets find out why?

If you aggregate the rail failure statistics in northern (or southern) latitudes you almost always see a seasonal pattern, with increased levels of broken rails occurring in the colder months of the year.

If you resolve the data still further you often see a spike in failures happening with or immediately after the first cold spell in the season.

Why?

Image from Sydney Trains twitter feed

This article aims to explain why rail failures are more common in the colder months. There are a few factors at play -

• Rail Stress
• Pre-existing defects
• Material properties

I'll try and cover each in a little detail. But first I must point out that rail failures are rare. In the UK there is around 10,000 miles of track (20,000 miles of rail), and currently under 100 rail failures recorded per year, (down from around 1000 just 20 years ago). So there is certainly no need to panic. Ok so lets explore why they are more likely to break in winter than summer. If you think I've missed a reason then let me know in the comments.

Rail Stress

Most new rail is continuously welded into long lengths many hundreds of metres long to provide smooth running and low maintenance track. However being so long poses some engineering challenges. When temperatures change objects change size very slightly (known as thermal expansion). For objects in our everyday life this change in size is imperceptible. However when you have a rail lets say 1km long the change in length becomes significant. A 20 deg C temperature change will alter the rail length by around 250mm. As rail temperatures in direct sunlight can often be double the ambient air temperature (think how hot your car roof gets on a sunny day) you can have significant changes in rail temperature and therefore changes in rail length to manage. In order to deal with the thermal expansion and contraction the rail is usually often stretched into position when it is fitted, leaving the rail under tension (stretched) for most of the time. Rails under tension are far easier to manage than those under compression. Compression in the rail can result in track buckles/heat kinks which pose a real risk to safety.

Photo of a heat induced rail buckle (heat kink) courtesy of Paul Fuller

The temperature where the rail stress changes from tension to compression is known as the "stress neutral temperature". Other names for the same thing may be the Stress-Free temperature, Rail Neutral temperature, and I am sure there are other names used around the world. Let me know in the comments which ones I've missed. Each network may have a different value depending on the climate in that area. Regions which experience very large seasonal temperature changes may actually have to re-stress their track between summer and winter. You can find more on rail stressing here (Wiki-Link)

So what has this got to do with the failure rate of rails? Put simply it is easier to break something that is under tension than something that isn't. The more tension applied the smaller the force or crack size needed to break the rail. Colder weather means the rails are under greater tensile stress and so are more likely to break. Hence failures are more likely in winter than in summer.

Pre-existing defects

Rails get defects in them - it is a fact of life. Over time and the passage of repeated traffic these defects may slowly grow. For example a fatigue crack may start from the surface and slowly grow over thousands if not millions of loading cycles as each wheel passes over it. As the crack slowly inches through the material there is less and less material left to support the load, so the crack growth may accelerate. Eventually there is not enough material left to support the load and "BANG" the rail breaks.

Fatigue related failure caused by an inclusion in the rail head

So what has this got to do with seasonal effects on failure rates? Remember in the previous section that in winter the rail is under greater tension? That increased tension makes cracks more likely to propagate to failure.

Imagine a crack has the whole spring, summer and autumn to grow slowly (a fatigue crack). In the first cold snap the rail tension increases, and that increase in tensile stress can be enough to accelerate the crack growth so that the rail fails. This is the main reason you tend to get a spike in rail failures around the first cold spell of the winter. The cracks that have been growing all year (or indeed those that survived the last winter or perhaps the one before) may have grown slowly over time but are now placed under bigger tensile forces. If the crack has grown large enough, the remaining rail cannot support the load and it fails.

Material properties

This is a small effect but I'll include it for completeness. As steel (and indeed most materials) gets colder the energy required to grow a crack through it decreases. Rail steels due to their composition generally have quite low impact energies, so the colder it is, the more likely it is for a wheel flat to cause a rail to break (through growing a pre-existing defect usually unless the wheel flat is massive). However the difference in impact energy between normal and cold temperatures is quite small; my old university professors words spring to mind "Twice bugger all is still bugger all".

I hope this helps explains why winter and indeed the first cold spell of winter is usually the worst time of year for broken rails.

If you think I've missed some other reasons then please let me know in the comments below and tag or share to those in your network who you think might benefit or be able to add more to it.

Follow the #rail101 tag for more content like this or there is a helpful index here

Last but not least if you need rails, or rail advice then contact me and I'll try to help.

Image from Network Rail Twitter feed