He said mediation was confidential.
"The strikes are still on but we will continue to see if we can nut it out."
No date had been set for further talks, Butson said, but he was "optimistic" of a settlement before strike action next month.
KiwiRail, in a statement attributed to acting chief executive Todd Moyle, said: "Negotiations remain challenging but we are still talking, and will continue to work towards a settlement."
KiwiRail workers have voted for a nationwide strike affecting all rail operations, including rail ferries, as well as the Auckland and Wellington Metro systems. The industrial action will take place on December 16 and 17 and affect both the North and South Islands.
Transport and freight sector commentators predict the first strike action by rail workers in more than 20 years will cause freight delays in an already-overburdened supply chain system by more than four days - with a knock-on effect for perhaps weeks.
Union general secretary Butson previously told the Herald he was hopeful that the sudden resignation of KiwiRail chief executive Greg Miller last week would result in a "different approach" to the mediation table.
"There was definitely a greater willingness to try to do a deal," he said.
Butson said KiwiRail's mediation team had been fronted by Moyle and KiwiRail "employment relations" boss Maryann Street, a unionist and former Labour MP.
"Mediation was adjourned and KiwiRail said they were going away to think about things."
Butson said it should be noted that the union had given plenty of notice of the strike action.
