The incident occurred at a level crossing in the town of Alt Duvenstedt. It was not immediately clear why the truck was on the track.
The train was partially derailed and the truck driver had fled before the collision, according to a statement by the federal police, Xinhua news agency reported.
Some 20 people were injured and among them three were in serious condition. A helicopter was dispatched to transfer one of the injured to hospital, according to the statement.
A large batch of rescue workers were on site to repair the damaged overhead lines.
Regional traffic was heavily affected by the accident and replacement buses were dispatched to carry the train passengers.