A passenger punched a fellow commuter in the head after being told not to stare at his laptop screen.
Police said the 46-year-old man was attacked after he noticed another passenger was looking at his computer screen.
He was working on his laptop and noticed the other passenger gazing at his screen, so he asked him not to.
As the victim went to move across the carriage, police said the attacker punched him in the head and then left the carriage.
The victim suffered bruising to his head and damage to his knee which he injured during the assault.
The attack took place on a train between Clapham Junction, south London, and Gatwick on November 10.