Police say two arrests have been made over an alleged incident at New Street Station.
British Transport Police said they were called to reports of an alleged fight onboard a train travelling from Manchester Piccadilly to Reading.
Officers say they met the train at New Street just before 5pm today, Wednesday, May 18.
They found a man was unconscious and he was taken to hospital by ambulance, police said.
Two men - aged 34 and 36 years - were arrested on suspicion of Section 18 Grievous Bodily Harm (GBH) with intent and inquiries into the incident are ongoing.
British Transport Police said in a statement: "At just before 5pm this evening officers were called to a report of a fight onboard a train. The train was travelling from Manchester Piccadilly to Reading.
"When officers met the train at Birmingham New Street, they found an unconscious man on board. The man was treated at the scene by West Midlands Ambulance Service medics and has been taken to hospital.
"Two men have been arrested. Officers continue to investigate the circumstances of the incident and would like to hear from anyone who was travelling on the train or has any information about the incident.
"Anyone with information is asked to contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 466 of 18/5/22. Alternatively, you can call Crimestoppers anonymously on 0800 555 111."