A man was reportedly pushed off a train at Herne Hill station last month, suffering a fractured elbow requiring surgery.
The incident took place shortly after 6pm during rush hour on Wednesday June 5.
Now British Transport Police have released CCTV pictures of a man they want to speak to in connection with their investigation.
The victim, a man aged in his 40s, was reported to have sustained his injuries after being pushed off the stationary train at Herne Hill following an argument.
He was helped back onto the train by fellow passengers, who asked the suspect to leave, said police.
The suspect was last seen at the platform at Herne Hill.
The alleged assault happened on the 6.13pm London Victoria to Beckenham Junction service.
Police asked anyone who recognises the man to contact them by texting 61016 or by calling 0800 40 50 40 and quoting reference 275 of 09/06/19.
Officers said they also want to speak to anyone who saw what happened.