He previously pleaded not guilty to attempting to murder Jonathan Gjoshe, Sachin Balakrishnan, David Presland, Scott Bletcher, Scott Green, Kevin Deely, Stephen Crean, Raza Aslam, Michael Paffett, and Samir Zitouni on the LNER service.
Alongside his guilty pleas to 12 counts of wounding with intent and two of attempting to wound, Williams also admitted affray at a barber shop in Peterborough and stealing knives from an Asda location in Stevenage.
Both offences took place on October 31, one day prior to the attack which he is accused of carrying out.
WIlliams also pleaded guilty to possessing a bladed article in a public place in Peterborough, Pontoon Dock DLR station at Silvertown, east London, and on the train between Peterborough and Huntingdon on the day of the mass stabbing.