Relatives have paid tribute to “devoted family man” Lee Pomeroy, 51, who was stabbed to death on a train in Surrey.
The married father of one from Guildford was on his way to London with his 14-year-old son for a day out.
In a statement his family called the attack, the day before his birthday, “horrific and pointless”.
A man, aged 35, was arrested on suspicion of murder and a 27-year-old woman was arrested on suspicion of assisting an offender.
The statement from the family on Saturday added: “He was a devoted family man and did everything for his family.
“He was an honourable man and would always help somebody who was in trouble.
“Lee would have had his birthday today, but did not get to see his presents.
“He was an honest, bright person, who loved music very much, he knew history and art and he was a bachelor of science in maths.”
Mr Pomeroy and his attacker both got on the train at London Road station in Guildford at about 13:00 GMT.
Det Ch Insp Sam Blackburn from British Transport Police, said: “This is a fast moving investigation.
“We are now confident to say that this is not believed to be a random assault.
“In the moments leading to the violent killing, both men appeared to be involved in an altercation lasting three minutes.
“Nothing justified the extraordinary violence that followed and we are concentrating our efforts on the on-going investigation.”
The suspect was arrest at about 06:00 on Saturday.
Assistant Chief Constable Sean O’Callaghan told BBC Breakfast the two men had moved through a carriage having a discussion for a few minutes before the “unprovoked violent attack” which saw Mr Pomeroy stabbed multiple times.
He said witness statements, support from other forces throughout the night and CCTV of the offence all led to the arrest.
Officers are still appealing for witnesses to come forward, but Mr O’Callaghan said police were “confident” in the arrests they had made and were not seeking anybody else in connection with the attack.
The father and son had been heading to London Waterloo, set to “spend some quality time together” in the city, relatives said.
More than 40 officers from British Transport Police are working alongside Surrey Police.
The forces were still searching for the murder weapon between Horsley and Farnham on Saturday.
A post-mortem examination is scheduled for early next week.