People who travel by train in Sydney are being urged to stay home tomorrow ahead of a snap strike by rail drivers.

The NSW Rail, Tram and Bus Union is refusing to staff any privately built trains on the network in protest of further privatisation of services.

They are also asking for a 3.5 per cent pay rise.

The industrial action will shut down 70 per cent of the city's trains, impacting workers but also students who have just returned to school.

"My advice is if you don't have to travel tomorrow please stay away from public transport," Howard Collins from Transport for NSW warned today.

"There will be delays – we've got a few extra buses on board, but you need thousands of buses to replace train services."

The strike is expected to cause significant delays and commuters are being asked to only use the network if it is essential.

The trains that are available will offer a basic level of service, with some running on a Sunday timetable.