A three-year-old boy was left alone on the platform in Hagen, North Rhine-Westphalia, while the train continued with his father.
The father of the family had left his son and suitcase on the platform on Saturday evening to get the stroller off the train, the police said on Sunday.
However, the doors closed and the train started again.
Meanwhile the boy stayed behind alone with the suitcase.
Federal police officers temporarily took the boy into custody at the police station until the father finally picked him up again.
kah/AFP/dpa