Trains will grind to a halt across Adelaide as drivers stop work in search of a better pay offer from the network's private operator.
Rail Tram and Bus Union members will strike for four hours on Thursday as relations with Keolis Downer curdle.
The union had initially planned a number of low-level industrial actions, including bans on workers checking tickets and reporting graffiti, but says it was left with no other option than to stop work entirely after the operator threatened legal action.
RTBU SA/NT secretary Darren Phillips said it was extraordinary Keolis Downer chose to escalate the enterprise agreement dispute in that way.
"It is clear ... that Keolis Downer does not care about the service it provides to the people of South Australia," he said.
"This is appalling behaviour by a ruthless, profit-driven multi-national and it just demonstrates once again why the privatisation of South Australia's public transport operations was such an unmitigated disaster."
Services are expected to stop running across all Adelaide lines between 9am and 3pm on Thursday.
Adelaide Metro said a small number of substitute bus services would run in close vicinity to typical train lines while services were stopped.
"Customers are encouraged to consider alternative transport arrangements, including catching regular bus services, trams, or organising a lift with family or friends," it said in a statement on Wednesday.
About 40,000 passengers use Adelaide's rail network each day, but as the stoppage is due to occur during school holidays and outside peak periods, disruption on Thursday should be relatively low.
However, further two-hour stoppages planned for Tuesday and Wednesday will coincide with morning and afternoon peaks, respectively, potentially causing commuter chaos.
In a statement, Keolis Downer apologised to passengers for inconvenience caused, maintaining it continued to negotiate with the RTBU in good faith.
The union says wages are the main sticking point in negotiations.
The RTBU is seeking a 20 per cent pay rise over four years, while Keolis Downer is offering 14.7 per cent increase over the same period.
"South Australian rail workers have received below-inflation pay rises for a number of years, and as such they have borne the brunt of the cost-of-living crisis," Mr Phillips said.
A government spokesperson said it wanted to see both sides resolve the matter quickly.
"These services will be back in public hands next year at which point the government will have an active role in future pay negotiations," the spokesperson said.
