The 28-year-old victim, from Liverpool, required surgery for his injuries following Thursday's attack.
He was travelling on the 20.34 GMT service from Liverpool Lime Street to Birmingham New Street when he challenged the group.
He was attacked after leaving the train at Winsford station, Cheshire, at around 21.10.
Det Sgt Matt Davies, of British Transport Police, said: "Thankfully the victim's injuries are not life-threatening, but he remains in hospital where he has received surgery for his wounds.
"Our investigation into who is behind this attack continues and we have been at the station conducting a forensic examination of the scene, local inquiries and reassurance patrols.
"It is vital we now speak to anyone who was on the 8.34pm train out of Liverpool and witnessed the disturbance in the carriage, as well as anyone else who was in the area of Winsford station at 21:.10 and saw what happened."
The attacker was in his late 20s and of stocky build. It is believed he was part of a group also consisting of four or five women.
He followed the victim before being repeatedly stabbing him in the groin and upper body.
The victim was taken to safety by another passenger from the train.

They are accused of conspiring together, and with others, to sell goods in 2012 that contained wholly, or in part, a mix of beef and horsemeat.
Ulrik Nielsen, Alex Ostler-Beech and Andronicos Sideras will appear at City of London Magistrates' Court on Tuesday, 27 September.
The charges follow an investigation led by the City of London police.
A man was stabbed in a station car park after challenging a group throwing objects around a train carriage.
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Three men have been charged with fraud for allegedly conspiring to sell horsemeat as beef.