The incident happened between 2.20pm and 3.35pm on Tuesday, 11 April, when an unknown man boarded a Transpennine Express service at York.
He sexually assaulted the 19-year-old victim before leaving the service at Manchester Victoria station.
Officers believe the man 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 354 of 11/04/2023.
Alternatively, call Crimestoppers anonymously on 0800 555 111.
