Tens of thousands of rail users faced disruption after members of Aslef and the Rail Maritime and Transport union in Scotland staged their latest 24-hour walkout.
Unions said there were no talks planned with management following the breakdown of negotiations last week.
AA Roadwatch reported heavier than usual traffic on Scotland's main roads during the morning rush hour.
The head of personnel at ScotRail, Donald Macpherson, said further talks with the unions were "imperative".
He told BBC Radio Scotland: "The main pressure we are aware of is from our customers who can't get into work today and have not been able to get into work on three previous days. "Clearly any escalation of strikes will be bad news for our customers, bad news for the rail industry in Scotland as a whole and ultimately bad news probably for our own staff."