In collaboration with the Cologne Public Transport (KVB), parts of the movie were shot on a -usually unaccessible- sublevel of the Cologne underground network. Those parts were shot in an old train exclusively provided for the shootings. The train was sold a week later to an Italian railway company. Thus it was clear that those scenes could not be re-shot later on. Luckily, it wasn't necessary either. The team had only 4 hours at their disposal down there to get everything in the can. Marie-Agnes Reintgen actually did walk into a real tunnel of the subway network. The traffic was carefully supervised during the time of the shot (midnight), so it was guaranteed she was not in danger.