The union and the NSW government are locked in a years-long industrial stalemate over the Korean-built intercity trains because of safety concerns.
The government has pledged to spend millions of dollars modifying the fleet, but the union wants assurances from the government in writing.
The union has stepped up industrial action this month, and on Wednesday strikes hit the Bankstown, East Hills and Southern lines in southwest Sydney.
Strikes on other segments of the network are planned on Tuesday and Thursday next week.
RTBU NSW secretary Alex Claassens said next week's action would go ahead unless something "brilliant comes floating down out of government".
Mr Claassens said the union was still waiting for written guarantees on safety issues related to the new fleet, and for a "proper conversation" with the government on wages and conditions.
"If we get a signed deed tomorrow it will be a miracle, but if we get one it'll have to be checked out," he told reporters at Central Station.
"If we do get it to a point we're comfortable with it and they're prepared to sign it then we would obviously send it out to our delegates."
He said a meeting between the union, government and Transport for NSW officials was scheduled for Thursday afternoon in a bid to break the deadlock.
Mr Claassens rejected government calls that an agreement on the train fleet should also address a new enterprise agreement.
"We've said consistently that we want the train fixed as a separate process," he said.
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.
