The 20-year-old was travelling on the 9.48pm service from Leeds on Saturday, April 14 when he was attacked just before the train reached Swinton, Rotherham.
EXCLUSIVE: Jailed Sheffield brothers speak out from prison after a decade behind bars for murderBritish Transport Police said he was assaulted by three men in the corridor near a toilet.
CRIME: Detectives still working to crack Sheffield murder caseThe assault left him with serious facial injuries, including a broken jaw, which required hospital treatment.
POLICE: Man with sword chased children from Sheffield parkOne of the men was white, in his twenties and around 6ft tall.
He was wearing a dark grey polo shirt.
Another of the men was believed to have been mixed race, with curly hair and was in his mid to late twenties.
He was wearing a black top.
A British Transport Police spokesman said: "Officers would like to speak to anyone who was on the train and may have seen what happened."
Anyone with information should call 0800 40 50 40 or text 61016 with reference 149 of April 15.
Crimestoppers can also be contacted, anonymously, on 0800 555 111.