The RMT said it had “no option” but to stage further 24-hour strikes on Sunday July 24 and 31 amid the failure of talks through the conciliation service Acas.
The union opposes any extension of driver-only trains and wants guards to be in control of operating doors.
It says safety was at the core of the dispute, which has already resulted in several days of industrial action.
ScotRail says 59 per cent of customers already travel on a train where the doors are safely opened and closed by the driver, and insists that the job, pay and conditions of conductors is guaranteed.
RMT general secretary Mick Cash said: “The entire trade union movement stands shoulder to shoulder with the ScotRail staff fighting for the basic principles of rail safety who once again showed their resilience and determination over the weekend in the latest phase of industrial action.
“Despite this series of rock-solid strikes, it is extraordinary that Abellio/ScotRail continue to ignore the strength of feeling amongst their staff over the extension of DOO (driver-only operation).
“That disgraceful and cavalier approach to jobs and safety on Scotland’s railways now leaves us with no choice but to put on additional strike dates.”
He added: “The union remains available for serious and meaningful talks.”