How Climate Change Affects Transportation Systems - Part 1: Heat

This article is part of a series on the effects that climate change has on transportation systems.

When you stroll around the city of Nuremberg in Bavaria, you see them everywhere: The tramways in the characteristic red-and-white livery. They are a vital part of the public transportation system and take you to popular places like the Reichsparteitagsgelände, the Dutzendteich and the Nuremberg Castle. Visitors and tourists alike rely on them every day to move around the city. Yet at the end of June 2026, services on the whole tramway network had to be suspended. They could only be fully resumed on July 8th. This was the longest interruption of services since World War II. What had happened?

Bitumen: The Black Dough

At the end of June, there was a weather condition rarely seen in Germany until now: An extended heat wave with temperatures up to 40°C. In Nuremberg, this caused temperatures at the street level to rise over 70°C.¹

Both the asphalt that the streets are made of and the tramway rails, which are made of steel, can handle this. But here comes the catch: There is also bitumen in the street. The bitumen is there to seal the space between the rails and the asphalt.² Without this seal, melting water could enter the underground, carrying road salt with it. That salt could then damage the foundation of the tracks. For this reason, a flexible sealing is needed, and that is bitumen. The bitumen must not become brittle during winter, and it is optimized for this condition. But in the heat wave, something unexpected happened: The bitumen melted.

From its usual almost solid state it changed to become a chewy lump, like dough in the kitchen. Like dough, it sticks to everything that touches it and is hard to get rid of. Sticking it did: It stuck to the wheels of cars and lorries that passed over it, spreading it across the streets and the tracks. It stuck to the wheels of tramways, wrapping itself around the wheels. And there it stayed.

When it cooled down a little bit, the bitumen became solid again. It clogged the wheels of tramways. It blocked the rails. In short, it was no longer possible to operate the tramway safely. All trams were called back to the workshop to prevent further damage to the vehicles and potential accidents. Operations had to be stopped.

Unsticking the Wheels

With all trams grounded in the workshop, the hard work to get them running again began. What nobody knew at that time was that it would take more than a week to get everything operational again.

The hard problem was to remove the bitumen. Once it hardened, it was very hard and extremely difficult to remove without destroying the wheels and rails.

It took a lot of experimentation to find effective methods to remove it all. One method that proved effective was to blast the wheels with dry ice. The ice particles at a temperature of -78°C make the bitumen brittle so that it can be removed, and they evaporate as soon as they hit their target.

Cleaning the rails was a lot of work as well. The VAG turned to a specialised machine that is used to clean away tire tracks from airport runways and race tracks.¹ But even here, a lot of labour went into cleaning the tracks and the switches, using dry ice, but also jackhammers and the good old scraper again.

All of this was only possible by the joint efforts of the tramway and subway maintenance personnel, joined by many volunteers. All of them were working under high pressure and in the stifling heat.

To get an impression of the effect of the bitumen, look at the pictures in this blog article by the VAG. And watch this video by the Bayerische Rundfunk. It shows the effects of the bitumen and the ordeal that the team of the VAG and many volunteers had to go through to get rid of it.

The hard work paid off. On July 15th, the whole tramway network was operational again.

Preventive Measures

The system outage could finally be fixed by the joint effort of many people. It had a huge impact on daily life in Nuremberg, as one of the pillars of the public transport network was affected. The VAG organized buses and taxis as a replacement, but these cannot match the capacity or speed of a tram, so we can consider this only a patch.

A solution has to be found.

The VAG would like to find a bitumen that does not melt in summer. Yet this collides with the requirement that it does not break in winter. A formula that fulfils both criteria has yet to be found. The VAG is cooperating with the Technical University of Darmstadt to find a solution.

In the meantime, the VAG has put some workarounds in place. When the temperatures are high, they now water the tracks and streets to keep them cooler. Also, the rail grinder of the VAG can be seen travelling the network throughout the day to quickly remove any patches of bitumen from the tracks before they solidify. And finally they are constantly monitoring the network, ready to close down traffic as a last resort in case the bitumen melts again. This way, they plan to prevent the bitumen from sticking to tracks and vehicles again, avoiding another extended outage.

We Need More Shade

As heat waves become more common, we need better ways to keep our tramways operational also in extreme weather.

Finding a bitumen formula that works both in winter and summer as the VAG wants to do will certainly help here.

But we also need to keep the cities and streets cooler. And here is a field where innovative solutions have yet to be found.

One obvious approach is planting more trees. However, planting more trees in a densely populated city is difficult, and especially when it comes to existing tram tracks. Trees take up space currently in use by streets and parking lots, which leads to usage conflicts and discussions. Their roots can collide with cables and pipes underground, so they cannot be planted everywhere. And their canopies must not collide with the catenary of the tram, restricting the places where they can grow and also making them less useful to provide shade where it is needed for the tram.

This means planting trees, while a good and necessary measure to protect cities against heat, will not fully solve the problem of shading streets and tram tracks. This is a challenge for city planners and engineers: How to shade our streets and keep them cool during heat waves? This would not only benefit tramways, but also pedestrians and the inhabitants of nearby buildings. If you have any suggestions or useful resources, please let me know in a comment.

Outlook

The outage of the tram network in Nuremberg shows the extreme effect that heat waves already have today on our transportation networks. Other cities like Leipzig were also affected.³ Climate change will make such events more common.

We need to prepare ourselves and our cities to prevent outages of the most basic infrastructure that we rely upon every day. Organisational changes and new bitumen can only do so much. New solutions for cooling and shading our cities will be needed.

The next article in this series will cover another increasingly common phenomenon: Flooding.

Footnotes