British Transport Police said the man pictured was on board a service travelling between Glasgow and Edinburgh when a disturbance broke out last year on July 19.
Officers believe he will be able to assist them with their enquires and have asked members of the public who may have information to come forward.
The 15-year-old, who was also on board the 9.23pm Glasgow Queen Street to Edinburgh Waverley Station service, was taken to hospital where she was treated for a serious injury suffered on-board.
A spokesman for the British Transport Police said: “Anyone who recognises the person in the image, or who has any information that will assist officers with their investigation is asked to contact BTP by texting 61016.
“Alternatively, Crimestoppers can be contacted anonymously on 0800 555 111.”