The train derailment occurred near the town of Riedlingen in the state of Baden-Württemberg, after extensive rain and storms in the area.
"The water triggered a landslide on the embankment next to the tracks, which in turn caused the derailment", writes the police in Ulm and the prosecutors in Ravensburg in a joint statement.
Around 100 passengers were on board when the accident occurred. At least 41 people were injured, some of them seriously.
The investigation into the incident continues, the authorities announce.
