Police have released a CCTV image of a man wanted in connection with a melee on an evening train between Nottingham and Swinderby.
The fight broke out at around 6.45pm on Saturday, July 23, between two groups of men.
The train was stopped at Swinderby and officers attended.
Sergeant Ian Wright said: “From the details that were taken by officers from people on the train on the night, and through reviewing CCTV and police body worn video cameras, we have identified nearly all the people involved in the melee.
“However we would like to also trace the man pictured, who we believe may be able to assist our investigation into this incident. If you recognise him, please contact us.”
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference 222 of September 2.
Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555 111.