CCTV footage shows the victim being attacked while he sat on the train from London Euston to Northampton
The victim was travelling from London Euston when he was approached by the attacker.
The footage shows the thug - who appears to be holding a beer bottle - talking to the victim and a woman.
As the train pulled into Milton Keynes station, the attacker can be seen punching the man in the face. This was despite other passengers being sat on the train, which was travelling to Northampton.
PC Alex Trivino said: “The train left London Euston at 11.24pm on Friday, 14 November, and an assault took place on board as it pulled into Milton Keynes around an hour later at 12.20am on Saturday, 15 November.
“The victim was travelling in a first class carriage. Throughout the journey, a man and a woman were talking to various other passengers in the carriage.
“As the train pulled into Milton Keynes, this man assaulted the victim, causing a cut underneath his left eye which required treatment at A&E.”
PC Trivino added: “I am confident that someone will recognise this man from 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.”