The victim, a 57-year-old man, travelled on the 6.20pm Northern Rail service from Newcastle on Saturday, March 11. It reached the station in the Northumberland town at about 6.40pm.
He had a verbal altercation with two or three men on the platform at Newcastle before he boarded the train.
On arrival at Morpeth, he became involved in a physical altercation with the same men. His injuries from this included a punctured lung and two broken ribs and he was in hospital for five days.
Officers would like to hear from anyone who was on board the train or at the station and may have seen or heard what happened.
If you have any information about the incident, call British Transport Police on 0800 405040, or text 61016, quoting reference number 129 12/3/17.