Trains in Sydney will run on an extended overnight timetable from midnight until Sunday, after the railway workers union issued an ultimatum to the state government as part of escalating industrial action.

The government capitulated to the Rail, Train and Bus Union's (RTBU) demand for 24-hour services to avoid a union threat to completely shut down the network. 

While that means there will be more services running in the early hours of the morning, the government is warning a stretched workforce could lead to delays to standard services during the day.

How will the Sydney trains timetable change impact me?

There is no detail about which train lines will be impacted by a potential reduction in services, but it is likely those travelling in the morning will bear the brunt. 

"We run a very real risk of delays from the night before rolling into morning peak commutes," NSW Transport Minister Jo Haylen said.

Light rail services will also be reduced throughout the same period impacting the Dulwich Hill (L1), Randwick (L2) and Kingsford (L3) lines.

Between 7am and 7pm, trams on the L1 Dulwich Hill line will run between Central and Lilyfield only, and trams on the L2 Randwick line will run between Randwick and Central only.

Both will run every 10 minutes and be replaced by buses outside of those hours.

Buses will also replace trams on the L1 line going the other way, between Dulwich Hill and Lilyfield.

No light rail will run on the L3 Kingsford line this weekend but replacement buses will run between Central and Kingsford between 5am and 1am.

Passengers are being warned to plan their trip and expect it to take longer.

Why are union workers doing more, not less?

While the 24-hour timetable is a result of industrial action, it is not a strike, nor is it a trial.

The union is hoping this weekend will build the case for a permanent 24-hour program for the train network throughout suburban Sydney and cement railway workers' importance to taxpayers.

RTBU NSW Secretary Toby Warnes said the demand for overnight train services formed part of rail workers' negotiations over pay and conditions.

"We are negotiating an enterprise agreement and we want to have the agreement landed before Christmas, it is our absolute goal," he said.

"That industrial action is designed to keep the pressure on leading up to that break so the government has incentive to come to the table and finalise our enterprise agreement."

He said railway workers were excited by the idea of running a 24/7 train network and said the union was encouraging its members to talk about the industrial action with passengers.

"They're excited about being able to provide this service to the people of Sydney," he said.

"It's important that members of the public know why they're receiving 24 hour public transport at the moment and the reason they're receiving 24 public transport is because we are taking the industrial action we are taking.

"We have pulled the government kicking and screaming."

Will Sydney trains run for 24 hours permanently in the future?

Sydney Trains chief executive Matt Longland has ruled out the idea of an overnight timetable becoming a permanent fixture due to the impact on essential track work and availability of the workforce.

"We're doing it this weekend to minimise inconvenience, we're cancelling track work in order to operate these services through the night". 

Timetable latest move in union scuffle

It is the latest tactic in the RTBU's ongoing stand-off with the government to secure better pay and conditions.

Among the union's demands are an eight per cent annual pay rise and a 35-hour working week.

In September, the union threatened to disrupt services unless the government agreed to bring in 50 cent fares, like those introduced in Queensland.

The government managed to resolve that dispute off by promising free rail fares across the network for a single weekend.

Having won the fight to operate 24-hour services on Thursday, Friday and Saturday nights, the RTBU is also introducing work bans that will progressively restrict the number of kilometres its members can operate train services on.

Ms Haylen said the combined effect of the union actions would be like a "boa constrictor" that would "eventually squeeze the life out of the network" as Sydney trains struggles to fill its rosters.