For almost three hours, the Stadtwerke Bonn (SWB) had set up a rail replacement service for line 61, according to press spokesperson Veronika John. However, the subsidence was repaired in the evening and the line was reopened. "No negative effects on the stability of the track are to be expected in the medium term due to the backfill material that was brought in," John emphasised.

However, as the investigation into the cause is still ongoing, a maximum speed of 15 kilometres per hour will be maintained on the affected section. "This may cause slight delays, but also otherwise the trains on Kölnstraße cannot travel at insanely fast speeds," the spokeswoman said.

How the hole came about was the subject of investigations by the city on Monday, which was in close contact with SWB, she said. According to the head of the civil engineering office, Peter Esch, employees tried to clarify the cause with a shaft camera. However, they did not have sufficient visibility. There are several sewers about six to seven metres under the asphalt surface, including a house connection and a rail sewer. Esch assumes that one of the sewers is leaking.

It is likely that such a leak would cause the soil above it to disappear into the pipe itself over a period of years, creating a cavity completely unnoticed. Now an expert should go down to find out what caused the subsidence. The investigation will take place at night so as not to interfere with railway traffic.

At best", the civil engineer said, it could come out that it is a dead channel that is no longer in use. In this case, it is not unlikely that the temporary filling of the hole by the public utility company will be sufficient and that no further work will be necessary. It is also conceivable, however, that the sewer is in use and needs to be repaired.

If it is a private house connection, the owner would have to pay for the damage. In the case of a rail sewer, the public utility would have to bear the costs, and in the case of a municipal sewer, the city itself. In this case, the repair could take several days and would probably be moved to the summer holidays to minimise the impact on passengers.

In recent years, there have been several incidents of this kind in various parts of the town. Most recently, as reported, on Löbestraße in Bad Godesberg, and before that at the Old Cemetery in Bonn. The civil engineering office does not keep records. However, Esch does not have the impression that there has been an increase recently. In midsummer, subsidence can occur more frequently. In the heat, asphalt becomes plastic, increasing the risk of pavement failure if there is a cavity underneath.

Leaking sewers, through which soil is washed out, are often a reason for cavities in the street. Another main reason - at least in Bonn - is rotten wood in the subsoil. "When building the Bonn underground, the construction workers used the technique of Berlin shoring," explains Esch. This involves securing the excavation pit with wood, which later simply remains in the excavation pit. Once the material has decomposed, cavities can develop - with consequences for the road surface as well.

Original text: Philipp Königs and Silke Elbern

Translation: Mareike Graepel