Two platforms at Euston Station have been cordoned off with paramedics rushing to the central London transport hub.
A British Transport Police spokesman said officers were called to Euston station at 11.48am on Friday following reports of a man seriously injured onboard a train.
Paramedics attended and the man has been taken to hospital for treatment.
The force added at this stage, it is believed he has sustained self-inflicted injuries.
A cordon is currently in place on platforms fourteen and fifteen.
A witness to the aftermath said the front two first-class carriages of the train pulling into platform 15 were cordoned off by police.
He told The Standard: “I just saw the police tape, with cops and 4-5 Avanti staff talking by some screens and paramedic bags.
“The train that was cordoned off was at Platform 15. It was the front 2 first-class carriages that were cordoned off.”