A pensioner suffered a fractured cheek on a train on Monday after a "frightening attack" near to Bromley Station.
The 81-year-old man, who was on his way to visit his grandchildren, was kicked and punched in the head, according to British Transport Police.
The grandfather, who had been travelling on a London Victoria to Orpington service, was attacked at around 6.30pm.
The attacker claimed the pensioner was "staring" at him and lashed out, according to investigating officer PC Clive Thomas.
He said: "A man boarded at Penge East with his girlfriend and a short while into the journey this man accused the 81-year-old victim of staring at him.
"Heated words were exchanged before the man jumped from his seat and delivered a fierce punch to the victim's head.
"He then went on to kick him in the head before leaving the train at Bromley South station with his girlfriend.
"This was a violent and frightening assault on an elderly man who was on his way to visit his grandchildren and has left him feeling shaken and vulnerable."
Officers who are trying to identify the man are now calling on members of the public to help.
They have released a CCTV image of a man they believe may have information about the attack.
Thomas said: "It is vitally important that we speak to the man pictured to find out what happened and I would urge him or anyone who recognises him to contact police immediately."