A security guard working for German rail operator Deutsche Bahn was critically injured after falling from a moving train during a violent confrontation with a passenger, police and prosecutors said.
The incident happened on Friday evening during a ticket inspection on a train travelling from Offenburg to Karlsruhe in south-western Germany.
A verbal dispute apparently broke out during the inspection, prompting two Deutsche Bahn security guards to be called in, according to the statement released early on Saturday.
A passenger believed to have been drunk allegedly insulted the two security guards. A physical altercation then broke out between the 36-year-old passenger and one of the guards, aged 26.
Both men fell to the floor during the struggle. A train door then opened for reasons that remain unclear, and the 26-year-old fell from the moving train near Ettlingen-Bruchhausen, police and prosecutors said.
It was not immediately clear how fast the train was travelling at the time.
The suspect was arrested while still on board.
The critically injured guard was eventually found beside the tracks during a large-scale search, around two kilometres from where the train later stopped.
He was taken to hospital for further treatment. No further information about his condition was available overnight.
Police are investigating the circumstances and exact sequence of events.
The train has been taken out of service while the investigation continues.