Firearms officers have scrambled to Queens Road Peckham train station after receiving reports of someone being “seriously assaulted”.
Officers from the British Transport Police were called at about 8pm on Saturday night after calls about someone being hurt on a train flooded in.
Police said a “thorough” search was carried out inside the station, before a man was found with a “slash injury”.
The man was rushed to hospital, officers added, but his injuries are not thought to be life-threatening.
An investigation has now been launched to understand how the man was hurt.
Severe delays were reported from Southern Rail and the London Overground while the incident happened.
A spokesman for BTP said there had been no arrests into the shocking attack, adding: “Anyone who was on the train or at Queens Road Peckham station and witnessed what happened is asked to contact BTP by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 602 of 8 December.”