Train passengers witnessing a murder did not know one of the men was holding a knife and stabbing another traveller to death with it, a court heard.
Darren Pencille, 35, denies murdering father Lee Pomeroy on a busy London-bound train on January 4.
The Old Bailey heard he has admitted possession of an offensive weapon and jurors saw CCTV of the gruesome attack on the unarmed dad.
The court heard passengers did not realise Pencille had pulled a knife from his pocket and was repeatedly knifing Mr Pomeroy as he tried to use his hands to defend himself.
Pencille plunged the blade into the 51-year-old's neck, opening up his jugular vein and carotid artery, before stabbing him another 17 times in the face, torso, thigh, arm and hands.
It was said the pair were shouting at each other after a disagreement over whether Pomeroy and his 14-year-old son were in Pencille's way as he moved up the train carriage.
His blood covered his clothing, it covered the area of the train where the assault took place.
All three had got on the train at London Road in Guildford, Surrey at around 1pm.
The knife attack started just five minutes later, it was said.
Train passengers witnessing the fight thought the men were punching each other as they grappled in the train carriage.
But prosecutors say Mr Pomeroy was defending himself with the punches while Pencille continued to stab him.
Prosecutor Jacob Hallam QC said: "The knife was up and in the same second it's out and up into Lee Pomeroy's neck in the same second.
"As Mr Pomeroy remonstrated with Mr Pencille - not hit or slapped or punched, we can see he doesn't do any of those things - Mr Pencille holding that knife in his right hand swung it and plunged it in the left side of Lee Pomeroy's neck.
"It is a blow that cut through Mr Pomeroy's jugular vein and his carotid artery which are the vital vessels that take blood to the brain.
"The people in the carriage saw what they thought was the two men exchanging punches.
"They didn't see the knife in Mr Pencille's hand. So they were half right, what they saw was Lee Pomeroy punching the first defendant trying to defend himself after he had been stabbed in the neck.
"Mr Pencille was not punching back, he was stabbing and he was stabbing Lee Pomeroy again, again and again and again.
"The 18 knife wounds found to Mr Pomeroy's body when examined in post-mortem a few days later provided the dreadful proof of the savagery of the assault that caused Lee Pomeroy that wound to his neck, eight wounds to his torso and other wounds to his arm, his hands and his thigh.
"All of those wounds were inflicted on him within seconds of this blow while Lee Pomeroy tried to defend himself.
"It was a fight that only one of them was going to win."
Jurors saw stills from CCTV footage on the train, which shows a pool of blood starting to flow around 16 seconds after the initial blow to the neck.
Mr Hallam said: "His blood covered his clothing, it covered the area of the train where the assault took place and the clothing of the first defendant.
"Sixteen seconds after the first blow you can see graphically the amount of blood that had come from Lee Pomeroy's body from this stage.
"When he was able to, Lee Pomeroy was able to lurch away from his attacker. He was never going to get far and he didn't, and he collapsed on the floor of that train."
The court heard the train reached Clandon train station at 1.06pm and Pencille gathered his belongings before fleeing the train.
It was said he dumped his hat, sunglasses and jacket in woodland before his girlfriend Chelsea Mitchell, 27, collected him.
Mr Hallam said: "Minutes after he killed a man he's talking to her and it's obvious what the two of them would have spoken about, what was at the forefront of his mind. He's killed somebody - what to do and how he needs her help."
Mitchell denies one count of assisting an offender.
The trial continues.
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