The suspect being sought over the stabbing of a man on a train in Surrey has been arrested along with another person, British Transport Police said.
The suspect had been on the run in Surrey following the fatal stabbing of a train passenger in front of his 14-year-old son.
The attack happened on the 12.58 service travelling between Guildford and London Waterloo stations on Friday.
Superintendent Paul Langley said: “What we know at the moment is that the victim boarded the train at around 1.00pm at London Road station in Guildford, along with his 14-year-old son.
“Traumatically, the victim’s son would have been nearby when his father was fatally wounded. This would have been an horrific and hugely traumatic event to have witnessed and we are providing him with as much support as possible.
“We know that the suspect also boarded the service at London Road station, however at this time we do not believe they are known to each other.”
Supt Langley said “violent events such as this on the rail network are extremely rare”, and there would be extra officers on duty on Saturday to help reassure concerned passengers and rail staff.
Officers were called to Horsley station at 1.15pm to reports of a stabbing on a train.
The victim was said to have suffered “multiple stab wounds” in the “frightening” attack.
The South East Coast Ambulance Service confirmed the man died at the scene.