Two tube passengers who launched a "vicious and unprovoked" attack on an innocent man between Pinner and Northwood stations are being sought by British Transport Police.
The assault happened after the 22-year-old victim, from Moor Park, boarded a northbound Metropolitan line service at Baker Street station and was sat listening to a personal music player.
Shortly after midnight, his attackers joined the carriage at Pinner and one of them argued with him for "looking at his friend".
The two continued to verbally abuse the man when one sat next to him and punched him in the face.
The victim tried to move away but was pushed back into his seat and hit by the second suspect, breaking his nose.
Police had been called but the pair fled the train at Northwood Hills station following the incident on Saturday May 10.
BTP's PC Alban Barnes, the investigating officer, said: "We have made extensive local enquiries, spoken with witnesses and reviewed CCTV from the station.
"We are also asking for help from any members of the public who may have witnessed the assault or who may have information that can assist our investigation.
"I am treating this incident very seriously. This was a vicious and unprovoked attack on a young man who was simply making his way home after a night out and I am asking anyone with any information about this assault, or who may recognise either of the men pictured in the CCTV images, to come forward and talk to us."
Anyone who has any information about the incident, or who recognises any of the men pictured, should contact the BTP on 0800 40 50 40 quoting background incident B5 of 08/07/2008 or Crimestoppers anonymously on 0800 555 111.