The attacker was also injured and taken to the hospital, police said.
Police did not provide any information on the suspect’s identity other than saying he was between 20 and 30 years old. They said his possible motives were under investigation.
Police said they were first alerted to the incident shortly before 3pm local time when several passengers on the train made emergency calls to police. Police said the train was stopped and the attacker was detained outside the train.
The interior minister of Schleswig-Holstein state, Sabine Suetterlin-Waack, voiced shock at the attack.
“It is terrible,” Suetterlin-Waack told German public broadcaster NDR.
“We are shocked and horrified that something like this has happened.”
Regional police and the federal police were on the scene and the prosecutor’s office was investigating the attack, NDR reported.
The train station in Brokstedt was closed for several hours and train traffic was delayed across northern Germany.
Train operator Deutsche Bahn expressed its condolences on Wednesday evening (this morning NZ), saying that “our deepest sympathy goes to the relatives of the victims. We wish those injured a speedy and complete recovery.” - AP