One of the reasons Sydney trains will grind to a halt this weekend is because of something fairly uncontroversial in other major global cities.

The NSW rail union has demanded the state government operate round-the-clock train services across Sydney from Friday to Sunday to avoid a work stoppage.

Transport Minister Jo Haylen said it was unfeasible for the government to continue operating overnight train services for maintenance reasons.

"We cannot jeopardise the maintenance of the rail system and we cannot jeopardise the services that people actually need to get to work in the morning and to get home."

Experts say introducing Sydney's 24-hour train service would be costly and require changes to operating and maintenance schedules.

Sydney's overnight trains ended 35 years ago

Sydney had a 24-hour train network up until 1989, when the Greiner Liberal government ended overnight train services.

University of Sydney senior lecturer of transport management Geoffrey Clifton said the overnight train services ceased because security costs were too high and maintenance was too difficult to undertake.

"Security costs were too expensive, they needed to have security guards on every train and every platform," Dr Clifton said.

"By running services overnight, they weren't able to do maintenance overnight and so they would have to schedule in more maintenance at other times of the day."

The trains were replaced by an overnight NightRide bus service from midnight to 4:30am, which still operates today.

Dr Clifton said Transport for NSW used the overnight to maintain its fleet of trains, tracks and stations.

"Currently we don't run 24-hour train services because the night times are used to … do all of those sorts of jobs you can't do when there's trains running through the network," he said.

He said the current night bus replacement services had been operating "quite successfully" for the past 35 years.

Other global cities have night trains

In 2016, Melbourne introduced 24-hour weekend train services which received strong patronage mostly from tourists, revellers and shift workers.

Public Transport Users Association spokesperson Daniel Bowen said the nightly services operated hourly and provided reassurance to people staying out late.

Mr Bowen said the services operated on all rail lines, except the smaller Stony Point line in Melbourne's south-east.

"[The trains] took over from an all night bus service that had previously been offered since the 1990s," Mr Bowen said.

"The problem with the buses was that they ran along quite different routes to the daytime trains so people weren't really familiar with the routes.

The London Underground offers a NightTube service that operates on Friday and Saturday nights on limited lines.

Transport for London said the overnight service cuts travel time for some Londoners by an average of 20 minutes, and in some cases, more than an hour.

New York operates a limited late-night service from midnight to 6am.

Mr Bowen said Australian cities should consider opening up night train services like Melbourne has.

"I think Australian cities are increasingly 24-hour cities. The idea that everyone is at home by midnight is no longer true," he said.

"There's plenty of people out after midnight not just at weekends but on other days of the week as well."

'You simply can't flick a switch'

Converting the current Sydney Trains schedule to include overnight services would not be easy, according to Dr Clifton.

He said the 150-year-old Sydney Trains network required constant maintenance as some parts of it have been in use since the 1800s.

Dr Clifton said an overnight train service would require more staff, security, money and a revamped schedule.

"It takes a lot of work to recalculate all the maintenance requirements and a lot of these maintenance events would have been scheduled months in advance."

Dr Clifton said introducing overnight services would require Sydney Trains to rewrite the entire train timetable and account for additional costs.

"The main issue is going to be how much will it cost, versus how much will the benefits be, and so that would be understanding how many people would be likely to use the service," he said.

"They'll need to know more about the maintenance impacts of running a 24-hour service because … rather than having seven nights a week to do maintenance, there will be five nights a week."

Nevertheless, Dr Clifton said the option of re-introducing weekend overnight services was something worth exploring.

He said the overnight services could revitalise the city's nightlife, make it easier for essential workers to travel around on late nights and create a "24-hour economy".

"Overnight train services, particularly on weekends, certainly will have benefits or make it easier and safer for people to return home after a night out," he said.

"They'll have more confidence in their ability to get around the city late at night, to speed up those journeys home late at night."