A witness in the same carriage described the wound as a deep gash that left his head with the side of his head hanging off.
Another passenger, who is in his 30s, did not want to be identified, described the "utterly frightening" moment when he saw a man using a machete that measured a foot long attack on the victim.
He was afraid he would be 'hacked underground to death' and said: 'All of sudden there was a huge disturbance.
"A man stood in front of the carriage, brandishing a black machete measuring six feet long.
"People began screaming, there was an uproar at the front carriage and people were pushed into the plastic and stuff.
According to the witness, he got into the next carriage from the attacker.
He said that the driver wasn't aware of any events for 10 minutes due to the intercom not working. Passenger were left defenseless on the Tube while the attacker 'taunted them'.
"The attacker ran down the train. He said that he heard screaming, but didn't know what had happened.
"He then stood at the other end of the carriage, taunting us, and making it seem that he was going down the carriage's door.
"It was absolutely terrifying. I thought for 10 minutes that I would be underground hacked to death.
Witness said that the Tube train finally stopped, and that armed police had arrested the attacker. He did not resist.
BTP reported that a man was arrested under suspicion of inflicting bodily injury. Green Park Station has been closed.
Paramedics were also on the scene and took the victim into a nearby hospital.
Police stated earlier that they were called at Green Park underground station in central London at 6.47pm this morning.
Paramedics were also present and the patient was taken to a nearby medical facility.
Station Green Park was evacuated, and it was closed.
One Twitter user wrote: "My daughter was in train behind-no announcements for 50 minutes while train stopped in tunnel, and passengers scared."
Another person described the 'alarms' and the long wait for police to arrive, and added: 'I was and am still scared.
BTP Superintendent Mark Lawrie tweeted that the station was shut down due to an incident at #greenpark but would be reopened soon.
"Victim being treated at hospital. Officers not injured, weapon recovered.