A young man died as a result of an "unprovoked" attack on board a train, after saying to his assailant "wagwan", a court has heard.
Graphic designer Rhys Stacey Parchment, 25, is standing trial at Blackfriars Crown Court charged with the manslaughter of 24-year-old Warsame Warsame, who was living at YMCA Surbiton.
Mr Warsame was found unconscious on a different train an hour later, and died in hospital from a bleed on the brain on March 4.
Opening the case for the prosecution on Wednesday, Zoe Johnson QC said: "In an unprovoked attack, this defendant punched Warsame Warsame twice in the head with so much force he fell to the floor.
"Although he was able to get up, he had suffered a catastrophic injury to the brain.
"An hour later he collapsed and subsequently died in hospital two days later."
The court heard how Mr Stacey Parchment had boarded the train at Surbiton, on his way to play football.
Mr Warsame boarded the train at Raynes Park, and walked through the carriage Mr Stacey Parchment was sitting in, he court heard.
Passengers reported hearing Mr Warsame say "wagwan" - a popular greeting spoken in a Jamaican patois - before Mr Stacey Parchment got up from his seat and confronted Mr Warsame, Miss Johnson said.
Mr Stacey Parchment punched Mr Warsame once in the head, sending his head back and striking an interconnecting carriage door, she claimed.
A second blow, also to the head, put Mr Warsame on the floor, it was claimed.
The prosecution claimed witnesses heard a female passenger tell Mr Stacey Parchment he should not have punched Mr Warsame.
Mr Stacey Parchment allegedly replied: "Yeah, well, he's a prick," before leaving the train at Wimbledon station.
Miss Johnson said Mr Warsame was confused and incoherent immediately after the attack, and repeatedly told passengers who tried to assist him: "I'm sorry, I'm sorry."
He also left the train at Wimbledon, before boarding another train towards Waterloo.
The jury was shown CCTV stills of Mr Warsame on board the train holding his head, before slumping across two seats.
But the court heard there was no CCTV of the fight itself, because a camera in the carriage was not working.
Miss Johnson said: "He has his head in his hands, he's fidgety, he lays across two seats and eventually collapses.
"The train arrives at Waterloo. Warsame does not move, and he remains lying there until the train leaves Waterloo, going back in the direction of Wimbledon."
Mr Warsame was found unconscious and snoring between Wimbledon and Raynes Park.
The court also heard how the 24-year-old had a pre-existing medical condition requiring regular dialysis.
He was also on anti-psychotic medication, following an attack inside a Kingston nightclub in 2007 that had altered his behaviour, the court heard.
But he had not taken the drugs the day before the attack, and had punched a fellow resident three times in the face earlier that morning, the jury heard.
Mr Stacey Parchment, of New Barns Avenue, Mitcham, denies manslaughter.
The trial continues.