Update
The T4 Line will be shut down completely for six hours today (Wednesday) and services at other times will be disrupted.
Sydney Trains updated earlier advice to say, due to protected industrial action, "trains will not run between 10am-4pm".
"Some services may also be cancelled or run to a reduced timetable from 6am-10am and from 4pm-6pm."
Earlier
Major disruptions are expected tomorrow on the the T4 Eastern Suburbs & Illawarra and South Coast rail lines due to protected industrial action.
Sydney Trains says services will be affected from about 6am, and the impact will be felt throughout the day.
Members of the Rail, Tram and Bus Union (RTBU) will hold a stop work meeting at Hurstville Entertainment Centre from 12pm - 2pm to support claims in relation to a new enterprise agreement.
Sydney Trains said there will be "major disruptions throughout the day, particularly on the T4 Eastern Suburbs & Illawarra and South Coast rail lines".
"A planned work stoppage will affect rail customers throughout the day on Wednesday, including in the morning and evening peaks, with fewer services. longer journey times, and changes to stopping patterns across the network," the statement said.
"While the union action officially starts at 10am, our customers will start to feel the effects from around 6am. To ensure the safety of our customers, staff, and network overall, trains will need to be taken back to stabling yards and depots before the industrial action starts.
"Although all staff will return to work at 4pm, customers will need to allow plenty of extra time while trains return to the network. The timetable is expected to be fully operational by around 8pm.
"In addition to Wednesday's stop work action, RTBU members are carrying out a range of other protected industrial actions which will progressively degrade service levels across the NSW rail network if they continue as planned."
Among a range of industrial actions taking place, Transport Officers have placed a ban on issuing fines and cautions until September 6.