Services to Inverness and Aberdeen from Glasgow and Edinburgh have been terminating at Perth this morning (Monday) due to a system issue there.
The disruption has also caused delays in the central belt, including to services between Glasgow and Edinburgh.
Train tickets are now being accepted on replacement buses.
The firm said the fault had been fixed by 7am, however, disruption was likely throughout the morning.
ScotRail had already confirmed a number of trains would be cancelled, mainly because of staff shortages.
Passengers are advised to check the ScotRail website for updates on rail services.