British Transport Police officers are appealing for witnesses after a man was punched in the face on a train between London Bridge and Eltham.
PC Clive Thomas, the investigating officer, said: "The incident took place on Thursday 21 October, shortly after the 9.40pm service had departed from London Bridge.
"The victim, who was making his way home, was sitting on the train when he was approached by two men, one of whom punched him in the face.
"The attack was only halted once other passengers on the train intervened and ejected the two men from the service."
The attack left the 39-year-old victim, from Eltham, requiring hospital treatment for injuries to his mouth and left him with cuts and bruising.
The first man is described as white and aged around 30 to 35-years old. He was tall, of slim build and had short, cropped hair.
The second man police want to speak to is also described as white, aged around 30 to 35-years of age and of slim build. He had cropped hair was slightly shorter than his acquaintance.
PC Thomas added: "We know from our inquiries so far that a number of passengers on the train kindly intervened by coming to the victim's aid and ejecting the two men from the train at either Blackheath or Lewisham stations.
"This was a completely unprovoked assault on a man who was simply trying to make his way home and anyone who recognises the men described are urged to come forward.
"We particularly want to hear from any passengers who intervened on the victim's behalf, or who witnessed this incident."
Anyone with information should contact British Transport Police on Freefone 0800 40 50 40 quoting incident B2/LSA of 21/10/2010.