A plan to shut down Melbourne's rail system next Friday has rattled the Andrews government, with ministers growing angrier over the Rail, Tram and Bus Union's erratic industrial campaigns against the city's train and tram operators.

On Thursday, the rail union shut down Melbourne's tram system for the first time in 18 years, hampering tens of thousands of commuters and leaving many stranded on a rainy Melbourne day.

Among them was off-duty nurse Laura Wells, caught unawares by the strike at a deserted stop in Fitzroy at the corner of Gertrude and Brunswick streets.

"I have just finished yoga and was going home to High Street in Northcote," Ms Wells said. "Now I will have to call my fiance or catch a cab. What a way to spend a day off. The strike is so annoying."

Less annoyed by the tram strike were taxi operators, who experienced one of their biggest days of the year.

Thursday's tram strike, taken after the Fair Work Commission approved protected industrial action, came after the union and operator Yarra Trams failed to agree on a new enterprise agreement.

The union is arguing for a pay rise of 19 per cent over four years for tram staff; Yarra Trams has offered between 13 and 15 per cent.

The workplace agreements between the tram and train operators and their workforces both expired in June.

On Thursday during the tram strike, the union escalated its industrial campaign against Metro Trains, vowing to hold a train strike at the same time next Friday, between 10am and 2pm.

Rail workers took the decision after Metro Trains told workers they would have their pay docked if they took part in other planned industrial action.

The union's state secretary Luba Grigorovitch said this was a threat, but a spokeswoman for Metro Trains said it was in fact a requirement under the Fair Work Act. "This is not a threat, this is a matter of complying with the law," she said.

Train controllers, station staff, authorised officers and other staff will stop work next Friday.

Metro has offered staff a pay rise of 14 per cent over four years, but the union says the entire enterprise agreement being offered by the rail operator is "not in its members' interests".

The union has approval from the Fair Work Commission for its members to also carry out industrial action such as temporary shutdowns - but it must give five clear working days' notice to the operator.

The escalation to a day-time rail strike will again hurt thousands of travellers, and exacerbate the rail union's open defiance of Premier Daniel Andrews, who on Thursday slammed the tram strike as unjustified and "unnecessary".

Mr Andrews said the government could take combined legal action with the public transport companies to stop more strike action.

"Why was there a need to take industrial action today, when you are scheduled to sit down and hopefully resolve these matters [on Friday]?" he asked.

But the RTBU's Ms Grigorovitch said the criticism was disappointing, and suggested the premier was ill-informed about what the union was fighting for.

"I'm sure that being the premier of Victoria, he's [Mr Andrews] probably not across the detail of the Metro Trains operations agreement, or the Yarra Trams operations agreement."

She said everybody - "the travelling public, the government and the company" - needed to be reminded that it was the legal right of its members to take protected industrial action.

Opposition Leader Matthew Guy said Thursday's tram strike was a throwback to the Cain-Kirner era in Victoria.

"Welcome to the 1980s: Labor governments talking about increasing debt. There's trams on strike, trains out next week and unions are playing cat-and-mouse," he said, predicting the RTBU was "going to create chaos and disruption ... and the state government appears utterly and totally incapable of doing anything about it".

- with Richard Willingham, Benjamin Preiss and Neelima Choahan