Talks restarted on Monday, but within hours Aslef issued a statement saying they had been delayed.
In recent weeks drivers have refused to work on their rest days, causing disruption and the issuing of a temporary timetable that cut more than 700 services across the network.
“Talks have been adjourned until later in the week. No further comment will be made until the talks are concluded,” said the union’s Scottish organiser Kevin Lindsay.
No reason has been given for the adjournment.
The new timetable resulted in the frequency of trains being cut and the times of last trains being brought forward.
Aslef’s executive committee last week rejected a 4.2% pay increase and urged the continuation of talks, while warning of a ballot on strike action if discussions were halted.
Last week, Mr Lindsay said he believed a deal was “close”, adding: “If we get an improved offer, and acceptable offer, there will be no strike action. It’s as simple as that.
“The ball is clearly in ScotRail’s court now, they can come up with an improved offer using the money that is already there in this package.
“Let’s get a deal sorted, let’s get the railway running, that is what we want.”