The incident took place on a TransPennine Manchester to Cleethorpes service at around 4.40pm on Wednesday, March 6.
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Police officers investigating the attack want to hear from witnesses to the incident, particularly a group of women who spoke to the victim at the time.
Call British Transport Police on 0800 405040 or texting 61016 and quote reference 435 of March 6.
Crimestoppers can also be contacted, anonymously, on 0800 555 111.