Police have been given more time to question a man and woman arrested over the killing of Lee Pomeroy on a train.
A 35-year-old suspect was held after a manhunt led police to a flat in Farnham, Surrey, in the early hours of Saturday morning.
British Transport Police said they had been given another 12 hours to question the man and woman, 27, who is accused of assisting an offender.
Mr Pomery was with his son when he was stabbed to death on Friday afternoon.
The father, who would have turned 52 on Saturday, was described by his family as “an honourable man and would always help somebody who was in trouble”.
Relatives have paid tribute to the “devoted family man” 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 “horrific and pointless”.
The family statement 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.”
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: “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 arrested 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 are 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.
A post-mortem examination is scheduled for early next week.