British Transport Police officers believe the man may have information which could assist inquiries regarding the incident which happened at around 3.10pm on Friday, March 20
The incident occurred as the woman was about to alight from the 14.37 Glasgow Central to Motherwell service.
Detective Constable Andrew Egan, the officer in charge of the enquiry, said: "I am appealing to anyone who recognises the man in the photograph to contact me as a matter of urgency."
Anyone with information should contact British Transport Police on 0800 40 50 40 or by texting 61016 – quoting reference B5 of 27/05/2014.
Information can also be given anonymously to the independent charity Crimestoppers on 0800 555 111.