Social media sites have circulated an alleged video of the attack in which a man is seen to approach two seated commuters before punching one of them, a man.
The woman, who is sat next to the victim, is then seen chasing the attacker off the tube and down the platform at Upton Park station.
Since the video emerged, a man has been arrested on suspicion of “assault occasioning ABH and using threatening/abusive words/behaviour or disorderly behaviour likely to cause harassment, alarm or distress”.
He has been bailed until 14 November.
Anyone with information is asked to contact British Transport Police on 0800 405 040 quoting ref 554 18/10/16.