Police are hunting for a thug who punched a commuter in the first class carriage of a train from London.
Detectives today released CCTV footage of the moments before the attack, which left the 41-year-old victim needing hospital treatment.
Before the assault, a man and a woman were seen talking to train passengers on the train after it left London Euston, police said.
The man then walked over to a passenger, who was sitting down in the carriage's far corner, and lashed out at him as the train reached Milton Keynes.
As well as the video clip released today, police have re-issued a CCTV picture of a man they want to question over the attack in the early hours of November 15.
PC Alex Trivino, who is leading the investigation for the British Transport Police, said: "I am confident that someone will recognise the man shown in these good quality moving images.
"I want to urgently speak to him about what happened. I would also like to hear from anyone else who was on the train, and saw what happened, and has not yet come forward to police."
Anyone with information can contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference MSUB/B4 of 16/03/2015 or call Crimestoppers on 0800 555 111.
