Police have launched an appeal after the man approached a 42-year-old woman as the train waited in Preston station and tried to engage her in conversation.
He then sexually assaulted her, before later being ejected from the Preston to Manchester Oxford Road service by the guard.
British Transport Police have released CCTV in relation to the incident, which happened on 28 September at approximately 6.50pm.
Officers believe the man in the CCTV image may have information which could help their enquiries.
If you recognise him, or witnessed the incident, please contact BTP by calling 0800 40 50 40 or texting 61016 quoting reference 596 of 02/10/18.