Anthony Williams also admitted possessing a knife on the service at Huntingdon last November.
The 33-year-old denies 14 charges of attempted murder and is due to stand trial next week.
The incident led to a man from Wimbledon being recognised for his bravery.
61 year old Stephen Crean stepped in to protect fellow passengers and sustained serious injuries to his left hand during the confrontation.
He received a Merton Heroes Award from the Mayor of Merton in recognition of his extraordinary courage.
