Commuters from the Illawarra and southern parts of Sydney will face greater crowding on trains and slower journey times to work during peak hours because some services will need to stop at inner-city stations left off the map for a new $20 billion metro line.
The state government has yet to reveal how it will service St Peters and Erskineville stations once the metro line opens. The stations are on three existing railway lines but only trains on the Bankstown Line stop to pick up and drop off passengers.
As part of the government's metro rail project, the existing line between Bankstown and Sydenham will be converted to carry single-deck, driverless trains.
The problem for residents of the inner-western suburbs is that the metro trains will not stop at St Peters or Erskineville.
It means the government will have to decide whether trains from southern parts of Sydney on the T2 South or T4 Eastern Suburbs and Illawarra lines will stop at the inner-west stations once the new metro line begins operating in 2024.
The predicament facing transport planners is that trains on both the T2 and T4 lines are already over crowded during peak hours in the mornings and evenings. Two extra stops risk lengthening journey times for commuters from Sydney's south and south-east.
The City of Sydney and the Inner West councils have been pushing for an extra station to be built on the metro line between Waterloo and Sydenham in order to service fast-growing populations at Erskineville and St Peters.
"The level of demand will only increase as development in this area continues," a City of Sydney spokeswoman said.
Apartments for about 6000 residents will be constructed at the Ashmore estate development at the border of Alexandria and Erskineville over the next decade.
Inner West Council said the government had yet to reveal how it intended to service St Peters and Erskineville stations once the Bankstown Line was converted to metro.
While the metro trains would boost frequency and capacity on the Bankstown Line, the council said it was concerned calls for extra stations on the new railway had been overlooked.
The council said any downgrading of rail services to St Peters would "significantly compromise" rezoning changes made in 2011, and the suburb's ability to "continue to operate as a vibrant, connected and liveable neighbourhood".
St Peters resident Cathie Hughes said overcrowding would worsen if trains on the T2 or T4 lines stopped at the two inner-city stations.
"We have no idea how our stations are going to be serviced," she said.
"The only answer is to have one of those other lines stopping at St Peters or Erskineville. But at peak hour they are packed."
Ms Hughes, who works in the CBD in an administrative role, said the government should be expanding the suburban rail network instead of converting an existing line.
"Why would you duplicate something that already exists when Sydney is crying out for heavy rail?" she asked.
Transport for NSW declined to reveal how it plans to service St Peters and Erskineville with rail once the metro line opens.
However, the transport authority said it would ensure that trains continued to service the two stations during the shutdown of the Bankstown Line to allow it to be converted to carry metro trains.
"The existing infrastructure allows for trains to continue to stop at Erskineville Station and St Peters Station," a spokesman said.
The conversion of the existing line also means thousands of commuters who catch trains on the Bankstown Line west of Bankstown Station risk losing direct services to Sydney's CBD.
The government has yet to reveal whether it will extend the metro line from Bankstown to Liverpool.
If it decides against an expansion, commuters who catch trains from stations such as Carramar, Villawood and Chester Hill will need to transfer between metro and suburban trains at Bankstown Station.
Transport for NSW said planning was under way on the delivery of rail services between Bankstown and Liverpool to cater for commuters once the metro line opens in 2024.
"Passenger loads will be regularly monitored to ensure the needs of customers are met," the spokesman said.
The authority said Sydenham Station's role as a key interchange on the network would become more important for people from the city's south and south west because it would be where they would switch to metro trains to reach destinations such as Barangaroo, North Sydney and Macquarie Park.
"This new interchange option delivers faster journeys and more options for customers and will help reduce crowding at CBD stations," the spokesman said.
"Converting the Bankstown line to Sydney Metro allows extra suburban trains to be allocated to other lines."