A man punched a fellow train passenger who tried to stop him abusing a man and a woman.
The thug accused a passenger who was sitting behind him on a London to Market Harborough service of pushing his chair.
He also shouted at a woman who was with the man after her mobile phone began ringing.
Pc Graham Sheer, of British Transport Police, said: "At this point, another passenger, a man from Market Harborough, interjected in a bid to stop the incident.
"The suspect then approached the man, pinned him back in his chair by grabbing the base of his neck and shoulder, and shouted at him before punching him in the face."
The victim got off the train at Market Harborough. He suffered bruises and scratches to his neck, shoulder and top lip.
The attack happened on a London St Pancras to Market Harborough service on Thursday, June 6, shortly after the train left the capital.
Pc Sheer added: "This was a frightening attack that left the victim feeling very shaken.
"All staff and customers have the right to travel on the rail network without fear of attack and we take these offences very seriously on the rare occasions when they occur."
The suspect is thought to be Irish and is white and in his 50s.
He is described as large, has balding grey hair and was wearing glasses and Lycra cycling clothes.
Contact British Transport Police on 0800 40 50 40 or text 61016 quoting reference B7/LNA of 19/06/2013.
Information can also be passed anonymously to Crimestoppers on 0800 555 111.
