The victim, a 27-year-old man, boarded the train in Leeds and sat opposite three men who also joined the 9.26pm Northern Rail service there.
Although the group and the man had initially been friendly, things turned hostile as the train arrived at Keighley.
One of the men ran up to the victim and punched him several times in the head, forcing him to fall to the floor.
Today British Transport Police released details of the incident, which took place just after 10pm on December 23.
Investigating officer PC Katalin Kelei said: “It is baffling why this man decided he would launch such a violent and unprovoked assault. Especially as the victim and him were having a friendly conversation just moments before.
“As you would imagine this attack has left the victim incredibly shaken, however I am pleased to say he did not sustain any lasting injuries.
“If you have any information which would help us trace the person responsible, then please do get in touch as soon as possible.”
Anyone with any information is asked to contact BTP by sending a text to 61016 or by calling 0800 40 50 40, quoting reference 291 of 19/01/2017.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.