The court heard how when a heated argument erupted between the pair, Mr Pencille shouted: “You touch me, you touch me and you see what happens at the next stop.”
Mr Pomeroy was said to have demanded an apology, saying: “You should not have humiliated me in front of my kid.”
At one point, Pencille was heard to say on his mobile phone: “I’m going to kill this man”, the court heard.
As Mr Pomeroy remonstrated, Mr Pencille swung the knife and plunged it into his neck, cutting through the jugular vein.
A jury deliberated for 19 and a half hours to find Mr Pencille, of no fixed abode, guilty of murder.
Following his conviction, British Transport Police Detective Chief Inspector Sam Blackburn described Mr Pencille as “devious and dangerous”.
He said: “He’s a dangerous man. Quite clearly with his previous convictions, where he also stabbed another man in the neck, and his propensity for carrying knives, he showed his dangerous, aggressive nature and that he wasn’t afraid to use that knife on that train on January 4.”