Commuters on the T8 Airport South Line were among those affected by major problems on the Sydney Trains network during the morning peak.
Wet weather and staff shortages were blamed for delays on five lines.
The Transport Management Centre said passengers travelling in both directions on the T1 North Shore, T2 Inner West and Leppington, T3 Bankstown, T5 Cumberland and T8 Airport South Line should allow plenty of extra travel time.
Rail replacement buses were provided to assist in some areas.
”Passengers are advised to allow plenty of extra travel time due to an operational issue,” a statement on Sydney Trains website said.