British Transport Police officers investigating the incident on the Sheffield-Scunthorpe service said it happened on the section of the journey between Doncaster and Scunthorpe and have now released CCTV images in connection with the incident.
They said a man boarded the train at around 5.40pm on Saturday, October 2, and sat on the aisle seat across from a woman. He engaged in conversation with her before leaning across the aisle and sexually assaulting her.
He then got off the train at Scunthorpe.
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Officers believe the man in the CCTV images may have information which could help their investigation.
If you recognise him, or have any information, contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 444 of 04/10/21.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111 with any information you have about the incident.