An agreement is still yet to be reached between the Rail, Tram and Bus Union (RTBU) and the state government over conditions, despite industrial action shutting down trains on every regional train line for one day in December.

RTBU Branch Secretary Luba Grigorovitch has called for a government-appointed mediator to help progress negotiations, which she said continued to go in circles.

"We're not getting anywhere, we need a circuit breaker," she said.

"We're trying to find middle ground but V/Line have demonstrated they are unwilling to compromise. We don't take any action lightly and ultimately would prefer it be avoided, but RTBU members are fed up and the public deserves a way forward."

A 24-hour stoppage will occur on the Geelong/Warrnambool lines on Wednesday, January 29.

V/Line chief executive James Pinder said it was "extremely disappointing and frustrating".

"Our passengers don't deserve to be bear the brunt of this decision by the RTBU," he said.

"Discussions with the RTBU took place as recently as last week, making this decision by the RTBU even more frustrating given talks have only just resumed for this year. We want to move forward with these discussions and urge the RTBU to do the same.

"We will be doing everything we can to minimise the impact of protected industrial action, to help ensure our passengers can get where they need to go and we sincerely thank them for their patience and understanding during this challenging time."

Ms Grigorovitch said the campaign was "all about conditions".

"Our members take great pride in their work, getting people from A to B, but when government and the company team up to attack their ability to deliver that service they take it very personally," she said.

A state government spokesperson said in December "we expect negotiations to continue in good faith between the parties involved".

The strike announcement comes after the Warrnambool line failed to reach its punctuality target of 92 per cent for the fifth consecutive month.

V/Line's December performance figures show 87.6 per cent of trains ran on-time last month - the worst result since May last year.

Rail operators said train and track faults, and the December strikes had impacted punctuality in the south-west.

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