The incident happened shortly after 3pm on Monday 10 April when the train conductor asked a group of young men to leave the service as they did not have tickets.
As they left the train, one of the group turned and punched the conductor in the face.
Officers believe the person in the images may have information that could help their investigation.
Anyone who knows him is asked to contact BTP by texting 61016, or by calling 0800 40 50 40 quoting reference number 340 of 10/04/23
Alternatively, call Crimestoppers anonymously on 0800 555 111.
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