Police want to hear from witnesses to a man doing martial arts moves and threatening passengers on a train between Sheffield and Leeds.
The incident took place at around 1.30pm on Saturday, December 13 on board an Arriva Cross Country train.
He is reported to have threatened passengers with violence.
A 27-year-old Sheffield man arrested in connection with the incident has been released on police bail until January 8.
PC Gary Reader, of British Transport Police, said: “This was completely unacceptable behaviour which left a number of passengers fearing for their safety.”
Anyone with information should call BTP on 0800 405040 or text on 61016, quoting PSUB/B6 of 19/12/2014.