In court, he pleaded guilty to three counts of robbery, three counts of attempted robbery and two public order offences.
He also admitted two public order offences, two counts of possessing a knife or bladed article in public, and common assault.
He will be sentenced at the same court on 16 October.
Kelt is already serving a three-year and nine-month prison sentence for three more robberies of items worth about £2,000, two more attempted robberies, two counts of possessing Class C drugs, and possessing a corrosive substance in a public place.
Det Sgt Steven Ridpath-Mitchell said: “The threats that he made against innocent commuters and the violence he used to rob them of their belongings is frankly sickening.
“There is absolutely no tolerance for robbery on the railway network.”