The Scotsman understands the Sunday stoppages are likely to continue until at least November, including during the Cop26 climate change conference in Glasgow.
Members of the Rail Maritime and Transport union (RMT) voted by more than 80 per cent to continue striking as part of their dispute over being paid less than drivers for working on days off.
Continued action was backed by 384 votes to 93, with 477 of the eligible 621 workers taking part in the ballot.
Industrial action short of a strike was supported by 429 votes to 48.
The RMT said its national executive committee was considering the result and a decision on further action would be announced shortly.
The vote was required by law after six months of the original ballot to permit action to continue.
RMT general secretary Mick Lynch said: "This is a stunning result and I want to congratulate our members who have stood firm for over six months in this fight for pay justice and equality in the face of outrageous provocation from the employer.
"Our campaign of action continues and it is now down to ScotRail and their political masters to take this key group of staff seriously, to stop snubbing talks and to get round the table with the union to negotiate a fair and just settlement."
The strikes have continued every Sunday since March 28, halting virtually all ScotRail services outside the Glasgow area.
The dispute was triggered by ScotRail agreeing with train drivers’ union Aslef to continue a previous deal for enhanced “rest day” working payments because the pandemic had forced the suspension of driver training to fill gaps caused by drivers retiring or moving to other jobs.
ScotRail said it was facing its most serious financial crisis because of the Covid pandemic and no extra money was available for the RMT’s demands.
A spokesperson said: “Industrial action is completely wrong at a time when we are seeing a gradual increase in the number of customers, but with passenger numbers at only 50 per cent of the pre-Covid level, this is not the time to put that recovery in jeopardy.
“Railway jobs are being put at risk by the reckless actions of the RMT and we are urging union bosses to think again and call off industrial action.”
The operator said it was only surviving through emergency taxpayer support of more than £400 million, which had nearly doubled such funding.
It said the lifeline had enabled the firm to avoid redundancies, furlough, wage cuts or staff benefits being reduced.
ScotRail said the 50 per cent increase in overtime payments sought by the RMT would see no additional hours worked.
It said in the current “stark environment” of passenger numbers and revenue still down, the top priority was recovery of the railway.
The operator said it appreciated “the hard work of everyone to keep key workers moving during the pandemic” but the focus should be on “making the railway an attractive travel option for passengers”.