British Transport Police (BTP) have released a CCTV image of a man they want to speak to over the incident on November 1.
The victim, a 24-year-old man from Middlesborough, was on platform 15 with two of his friends at about 7.45pm.
Detective Constable Tom Eastwood said: “He was approached from behind by a man who then punched him once in the face causing him to fall to the ground where he was knocked unconscious.”
The attacker ran off and boarded a train to Manchester Victoria.
Det Con Eastwood added: “The assault resulted in the victim’s jaw being broken in two places. This is completely unacceptable behaviour and we must trace the person responsible. We believe the man pictured may be able to assist with our enquiries into this incident. If you recognise him please contact us.”
Call 0800 405040, or text 61016, quoting reference PSUB/B5 of November 18.