Police were called to Wembley Park Underground station around 10.45am today (August 21).
Officers arrived and an injured man was taken to hospital.
His medical condition is still being assessed.
There have been no arrests and police are appealing to the public for information.
Wembley Park station was closed after the incident with severe delays to the Jubilee line.
A British Transport Police spokesperson said: “Officers were called to Wembley Park Underground station at around 10.45am today (21 August) following reports of a man being assaulted on a train.
“Officers attended and a person has been taken to hospital with injuries that are still being assessed.
“Enquiries are ongoing, and anyone with information is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40, quoting reference 228 of 21 August.”
