A man was left with a broken nose and another with a broken tooth after they were targeted along with a number of other passengers by a group of men on a Tulse Hill train.
British Transport Police are now circulating images of men they believe can help the investigation.
They say the unprovoked incident happened between Tulse Hill and Tattenham Corner stations on Sunday, February 10, at 7.42am.
Any witnesses, or anyone with information, should text 61016 or call 0800 40 50 40.
To give information anonymously, call Crimestoppers on 0800 555 111.