A 30-year-old man suffered a broken jaw after clashing with a fellow passenger over an empty seat on the Jubilee Line.
British Transport Police are appealing for more information about the incident, which took place on the 6.30am westbound service on Monday, June 28, at Canning Town.
A 43-year-old male was arrested on suspicion of GBH and released on police bail.
Officers said fight was over a vacant seat on board the train.
DC Michael Blackburn said they needed more information from people on the scene.
He said: “I am keen to speak to anyone who was on board this service and who may have witnessed the incident.
“The victim suffered a nasty injury and I urge anyone to come forward if they have any information which could help in the investigation.”
Those with information should contact British Transport Police on 0800 40 50 40, or text 61016, quoting background reference T-SUB/B3 of 31/07/15.
Otherwise, call Crimestoppers on 0800 555 111.