The IT consultant, 51, was stabbed 18 times in 25 seconds by 36-year-old Darren Pencille, the Old Bailey heard today.
Jurors sat in silence as they were shown graphic video of the incident on a Guildford to London train on January 4.
Pencille looked straight ahead in the dock, avoiding screens set up to show the footage.
Prosecutor Jake Hallam QC said: “The incident lasted 25 seconds and during those 25 seconds Mr Pomeroy sustained 18 stab injuries.”
British Transport Police Detective Constable Marc Farmer replied: “Yes, that’s correct. It was a quick and frenzied attack.”
Jurors saw images showing Mr Pomeroy and his 14-year-old son arriving at London Road station in Guildford and collecting their tickets before heading to platform two.
Shortly afterwards, Pencille was dropped off by his girlfriend Chelsea Mitchell and boarded the 1.01pm train to Waterloo.
He walked back down the carriage as Mr Pomeroy and his son were coming the other way.
It is claimed a row began over blocking the aisle and quickly escalated.
Mr Pomeroy was shown getting up and following Pencille into the next carriage and stood with his hands in his pockets during the argument.
The court heard how the defendant appeared to gesture with his hands and make a call, allegedly to his girlfriend, to say: “I’m going to kill this man. He’ll be dead.”
Pencille, who has admitted possessing a knife, allegedly struck out at Mr Pomeroy’s neck then continued to stab him as Mr Pomeroy attempted to defend himself.
The wound to Mr Pomeroy’s neck opened his jugular vein and carotid artery.
Other passengers could be seen leaving the carriage as blood pooled around the door area where the two men were grappling.
As Mr Pomeroy staggered away from Pencille he collapsed into a seat and could be seen panting heavily.
Afterwards, Pencille is seen in the footage bending down and picking up his sunglasses and mobile phone before fleeing the train.
Jurors have heard how he was picked up at Clandon station by 27-year-old Mitchell and taken to her home.
Pencille, of no fixed address, has denied murder, and Mitchell, of Farnham, Surrey, has pleaded not guilty to assisting an offender.
Afterwards, Pencille is seen in the footage beinding down and picking up his sunglasses and mobile phone before fleeing the train.