Officers were called to Southend station at 11.54pm on 22 October, following reports a male had received multiple stab wounds on board a service between Southend and Westcliff.
Colleagues from Essex Police and the South East Coast Ambulance Service also attended, and a 19-year-old man was taken to hospital where he remains in a serious but stable condition.
His injuries are thought to be life changing, and detectives continue to make enquiries to establish the circumstances of what happened.
DCI Sam Blackburn said: "This was a targeted attack and we are putting all our efforts into bringing the perpetrators to justice.
"I appeal to anyone who witnessed the incident, or the events leading up to it, to get in touch with BTP as soon as possible.”
Anyone who witnessed the incident or has any information is asked to contact BTP by texting 61016 or calling 0800 40 50 40 and quoting reference 687 of 22/10/18.
Alternatively, you can call Crimestoppers on 0800 555 111.