The railway and transport union North mourned the loss of a member in a post on its Facebook page on Thursday evening after the train attack in Brokstedt.
One trainee was killed and a colleague seriously injured in the attack.
The 19-year-old worked in DB vehicle maintenance in Neumünster, the branch manager told the German Press Agency on Friday.
Neumünster/Brokstedt – The 19-year-old and a 17-year-old were killed in the attack on a regional train from Kiel to Hamburg.
They knew each other and both attended the commercial school of the city of Neumünster.
Five other travelers were injured, some seriously.
The attacker was arrested by the police at Brokstedt train station after other passengers overpowered him.
An arrest warrant has already been issued for the 33-year-old.
Ibrahim A. was released from the Billwerder prison, where he was in custody for a violent crime, just a few days ago by order of the Hamburg Regional Court.
dpa