The RTBU is locked in a long-running stoush with the government over the controversial fleet, arguing the Perrottet administration has refused to sign a deal with the union that locks in fixes to safety issues raised by drivers.
As part of the fight, the union is taking industrial action on Friday, taking out 70 per cent of Sydney's rail capacity and set to cause commuter chaos.
RTBU state secretary Alex Claassens on Friday morning said concrete action was needed from the government before the union moved on its industrial campaign.
While the government has signalled it could be prepared to spend $264 million to modify the fleet, the union is accusing it of stonewalling on signing a written deal.
Mr Claasens said union action was "going to continue to escalate until such time as we have got a signed deed in our hand".
"Then we can then go back to the negotiating table to do our conditions and our wages which we haven't even talked about yet," Mr Claasens told ABC television.
"(We) have had no conversations about those wages at all ...There is a long way to go yet."
The union argues that a written document is needed after previous government offers on the fleet were followed by backflips due to internal politics.
During a meeting with the government on Thursday, elements of the modification offer were taken off the table, concerning union members that another government backtrack loomed, the union has said.
"My members will not listen to the 'trust me' moment anymore, we need to see a document," Mr Claasens said.
Protected industrial action on Friday will see train drivers refuse to operate foreign or privately made trains, meaning 30 per cent of Sydney Trains and NSW TrainLink services will operate on a weekend timetable.
Sydney commuters have been advised to expect significant train delays and cancellations.
Transport Minister David Elliott insisted the fleet was safe, but confirmed the union would see a written deal on Friday acceding to its demands.
"That is the outrageous situation that the union movement and the Labor Party have put us into," Mr Elliott told the ABC.
"The independent safety regulator has said there are no safety concerns for these trains."
Earlier this week Mr Elliott said the government had offered railway workers $3000 bonuses to return to work, which the union has described as "bribes".
