The prospect of further disruption came as passengers suffered a fourth day of cancellations and reduced services yesterday.
More than 250 trains - one in four - were halted. Some lines shut while services on others operated less frequently than normal and on reduced hours.
It followed similar disruption last Tuesday, Thursday and Saturday, with further strikes already announced for the next three Sundays.
RMT Scottish organiser Mick Hogg told The Scotsman there could be more to come if progress was not made in the dispute, over plans to downgrade conductors’ roles on a new fleet of trains.
He said: “The strikes are having an impact and the people of Scotland are suffering, so it is in everybody’s interest to get round the table and start negotiating seriously and properly. If the talks break down, we will consider further [strike] dates.
“I hope there are good prospects for progress but I’m not going to hold my breath.”
The fresh talks come a week after negotiations at Acas broke up without agreement.
ScotRail wants to switch control of doors from conductors to drivers on a new electric fleet due to start running on the main Edinburgh-Glasgow route and other Central Belt lines from next year.
Lower-paid ticket examiners would take over fare collection duties on some services.
ScotRail said the moves would save money and speed up trains, pointing out more than half its trains operate in this way already.
However, the RMT opposes the change on safety grounds.
Mr Hogg said he would not discuss separate proposals by ScotRail to firm up pledges over conductors on other trains, including a refurbished 1970s diesel fleet known as “High Speed Trains”. They are due to take over ScotRail inter-city services from 2018.
He said: “They are just trying to separate and split guards [conductors].”
A ScotRail spokesman said yesterday: “Tomorrow, we will resume our talks with the RMT. We still maintain there is a solution to be found to this wholly unnecessary strike.
“If the RMT will call off their action, we can get round the table and find a way to continue to modernise Scotland’s railway. We will also start the formal consultation on our new High Speed and diesel fleets. This will give certainty to around a third of our conductors.”