An $880 million investment will digitise NSW's train signal system and allow more trains onto the network, the State Government says, with all lines set to receive the new technology by "the mid 2020s".

The investment, announced by NSW Premier Gladys Berejiklian on Sunday, will see "Paris and London" technology come to Sydney and could give the network the capacity to one day run trains every 90 seconds.

Describing it as just as important as the invention of the double-decker train, Sydney Trains chief executive Howard Collins said the rollout of the signal technology would prepare the network for "the next generation".

And though Mr Collins said a train every 90 seconds was not currently required in Sydney, the technology had the capacity to achieve it.

Sydney Trains only met its 92 per cent punctuality target on 10 weekdays in May. ( AAP: Glenn Campbell )

"This technology has been proven," he said.

"In London we improved the capacity by 30 per cent when we saw a population explosion there, and here in Sydney we're seeing the same growth.

"So we had to do something about it.

"It's as important as the invention of the double-decker train.

"It ensures we have the opportunity to grow the network for the next 50, 60 to 100 years because it frees up this restriction on the number of signals we've got and the way this network has operated.

"I've seen other networks where there's two and a half minutes between trains, with this technology in some countries you have a train every 90 seconds — we might not need that yet, but we have this opportunity to use this technology for the future."

Which lines will be affected first?

In its initial rollout, the NSW Government said eight more services an hour would be added to the T8 Airport Line at the International, Domestic, Mascot and Green Square stations.

That meant trains on average every four minutes instead every six, with upgrade delivered in stages

Ms Berejiklian said it would enable a "turn up and go" system for the Sydney network, with services coming online progressively from the early 2020s.

Sydney's train network is often plagued with delays. This photo was taken in January. ( ABC News: Clare Blumer )

Sydney Trains missing punctuality targets

The Sydney network has been plagued with delays, frustrating commuters as Sydney grapples with a population explosion and under-pressure infrastructure — the Sydney Trains signal system is often blamed for the delays.

Just last week Sydney Trains revealed during May it reached its 92 per cent target of trains running on time just 10 out of 23 weekdays.

Its punctuality target is for 92 per cent of trains to arrive and depart within five minutes of the scheduled timetable in the Sydney CBD between 6:00am and 10:00am, and between 3:00pm and 7:00pm.

Not everyone welcomed the announcement.

NSW Opposition Leader Luke Foley described it as a "farcical" announcement, as more trains needed more drivers.

"The system has melted down because there's simply not enough train drivers employed," he said. "They can't even run the current train timetable."

"The Government has been asleep at the wheel when it comes to the training of new drivers."