Train commuters are being advised on Wednesday to work from home or find alternate transport as industrial action again impacts services.
The strike impacts the Bankstown, East Hills and Southern lines in southwest Sydney, but Transport for NSW said other lines were also affected.
The action is part of a years-long dispute between the Rail, Tram and Bus Union and the government over a fleet of Korean-built trains the union says are not safe.
The government has pledged to modify the fleet but the union wants assurances from the government in writing.
The union is stepping up industrial action this month as part of its campaign.
RTBU NSW secretary Alex Claassens said the action would continue unless transport authorities signed a deed to resolve the issues.
"We have notified actions, they're on our website for the world to see," Mr Claassens told ABC TV on Wednesday morning.
Strikes at this stage would also occur on Tuesday and Thursday next week, taking "different sections of the network out," Mr Claassens said.
The RTBU is scheduled to provide an update on future actions at 3pm.
Wednesday's strike has seen reduced service run on the T2 Inner West & Leppington, T3 Bankstown, T8 Airport & South, and the Southern Highlands lines.
The T4 Eastern and Illawarra line was impacted by a similar strike last week.
