Researchers have now discovered one of the main causes of railway track defects

"For the first two years of the project, I was somehow lost in so many track defects. I had to find out which one was the most critical for the Norwegian railway," says researcher Rui Tao.

A railway track can cause train accidents in dozens of different ways.

Cracks in the components that fasten the rails to the sleepers and uneven settlement deep in the substructure can have serious consequences. But which defect is the most critical?

The literature pointed in every direction at once.

The researcher worked like an investigator

Tao came to Norway in August 2021 with a background in water conservancy and hydropower engineering from Hohai University and Wuhan University in China.

He had little prior knowledge of the Norwegian railway system. The way he eventually approached the problem was inspired by how an investigator works on a criminal case.

"The process is really like the way investigators work in a detective novel. You have some clues, and then you try to find out who is behind it," he explains.

The clues had to be gathered from three fields that rarely communicate with one another:

• geotechnical engineering (the study of ground conditions)

• geomatics (the discipline that uses satellites and advanced measurements to map terrain)

• railway engineering

131 defects along 50 kilometres

Bane NOR operates the Roger 1000 measurement train. It records track geometry as it runs and covers the section between Trondheim and Støren twice a year.

Tao used data from the measurement vehicle to analyse vertical movements in the railway track from 2016 to 2020.

Along the 50-kilometre-long section, he found 131 isolated defects. Almost 70 per cent of them were heaving, not settlement. Around 70 per cent of the heaving defects returned after the track had been maintained.

Degradation was typically between zero and three millimetres per year, and in extreme cases six to seven millimetres.

The fact that heaving defects keep returning says something about what maintenance actually achieves, Tao points out.

"The maintenance is mainly done with the tamping machine, which corrects the geometry at the surface. If the cause lies in the substructure, the problem comes back again and again and again," he explains.

To quantify how serious a heaving defect is, he simulated an NSB Class 73 train running over modelled track defects.

The pipe acts as a tunnel for cold air

According to the researcher, two quantities are critical. If the contact force between the wheel and rail falls towards zero, the wheel loses contact with the rail and the risk of derailment increases.

If the vertical acceleration increases, passengers feel it as a jolt. The simulations showed that the heaving defects both increase operational risk and reduce passenger comfort.

 Tao then inspected the areas affected by heaving himself.

"We found that the locations affected by heaving coincided with culverts, and it was not random.  Almost everywhere the track had heaved, there was a drainage channel, a culvert, or another structure carrying water," he says.

A culvert is an open-ended pipe that carries water through the railway embankment. They are very common in Norway, and many of them were originally installed to drain farmland and channel rainfall into rivers and streams.

Built a scaled-down version

In winter, those open ends draw cold air through the pipe. This causes frost to penetrate significantly deeper into the embankment than it otherwise would.

This creates the three conditions necessary for frost heave: temperatures below zero, frost-susceptible soil, and enough water.

"In Norway, we have long winter and a lot of frost-susceptible soil, and the groundwater table is often shallow. Unfortunately, all three conditions are easily met along the railway lines," Tao points out.

But two things occurring together are not the same as one causing the other.

The field observations showed where the heaving defects were, not what happens in the soil around a culvert over the course of a winter. Tao needed to find that out in order to move forward.

Installing measuring instruments in an operational culvert is close to impossible. Tao therefore built a scaled-down version in a freezer room at NTNU.

There, the air above ground and inside the pipe was minus six degrees, while the water reservoir below was plus six degrees. Sensors measured displacement at the surface and temperatures at several depths.

The budget was NOK 10,000, or just over 1,000 USD.

"That's nothing. I almost had to mobilise everyone in the lab to make it happen," he says.

Researcher used condoms as membranes

The soil in the model was fully saturated, and water could not be allowed to enter the pipe itself. The solution was a latex membrane at each end.

"We used the cheapest solution we could find," says Tao. He collected almost 20 condoms from the free supply in NTNU's main building.

The experiment showed why a pipe can lift a railway track. When water freezes, it expands by around nine per cent. But this increase alone does not produce a large heave.

The difference is that frozen soil retains more water. Around each soil grain lies a thin film of water that does not freeze, and it migrates upwards towards the boundary between frozen and unfrozen ground.

When the water freezes into ice there, flat layers known as ice lenses are formed. They keep thickening as long as water arrives from below.

"The final conclusion is that ice lenses are the main cause of frost heave. Their growth could be seen with the naked eye during the physical model test," says Tao.

Only insulation works

Plans to increase drainage capacity by installing new culverts along Norwegian railway lines should therefore take these findings into account, Tao believes. 

His research shows that frost protection should be an integral part of the design of new railway projects in order to avoid frost heave.

Tao tested three different measures in a computer model. Only the thermal insulation worked. It reduced the unevenness that makes the track bumpy by up to 90 per cent.

The other measures had very little effect.

"Our conclusion is that only the thermal solution works. The problem with the hydraulic solutions is that the Norwegian winter is too long. They have some effect  at first, but over time, with a lot of cold, the heave returns to its original level. The mechanical solution also fails because the difference in stiffness between the geogrid and the frozen soil is too small for the geogrid to be effective," says Tao.

Satellites that watch the track move

Tao's research findings are most useful for new railway constructions.

For track already in place, digging up the embankment to install insulation would be too expensive. There, the heaving has to be kept down using tamping machines. Inspections determine where and when they are deployed. The need for inspections already exceeds existing capacity, Tao points out.

"We need better guidance for maintenance so that people are sent to the right place before a major problem or an incident occurs," the researcher says.

He therefore tested the PS-InSAR method. It measures ground motion by comparing radar images from satellites.

Although the technique is widely used to monitor soil moisture and flooding, it is rarely used on railways. Tao was unsure whether it would work under Norwegian conditions.

"Whether it could be used on the railway, and on the Norwegian one in particular, we had no idea," he says.

The real measure of success

All the measurements Tao collected required extensive processing. This was because measurement points were often incorrectly positioned, for example in the Gaula River instead of on the railway bridge deck.

Tao therefore developed his own method for removing noise and correcting the measurement points.

Along a 52-kilometre section, 4 of 7 anomalies matched incidents reported by train crews. 2 of the remaining anomalies were confirmed by data from the measurement train.

The final discrepancy was located in an area with sparse coverage.

The method can therefore provide warnings of developing track defects, but there is still a long way to go from scientific success to practical implementation by organisations such as Bane NOR.

"How it can be used in daily work is perhaps what I care most about. The true measure of our research lies not in its publication, but in its application to real-world challenges,” says Rui Tao.

References:

Tao et al. Evolution of isolated heaving defects for vertical rail displacements: a multi-disciplinary study, International Journal of Rail Transportation, 2026. DOI: 10.1080/23248378.2025.2469302

Tao et al. Monitoring of ground displacement-induced railway anomalies using PS-InSAR techniques, Measurement, 2025. DOI: 10.1016/j.measurement.2025.116863

Read a summary of the research project on the Norwegian Research Information Repository's website.

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