The biggest overhaul of Sydney's rail timetable in four years will result in more frequent trains for tens of thousands of commuters but for some it will mean longer travel times because they will have to switch services to get to their destinations.

When the timetable changes kick in next Sunday, those travelling from Campbelltown in Sydney's south-west to Parramatta, Granville, Strathfield and other stations on the Western Line will have to change trains at Glenfield. That is because T2 services will start from Leppington instead of Campbelltown.

And, while extra services will be added to the Bankstown Line quickening journeys for many, trains will skip more stations during peak hours. St Peters in the inner west, in particular, will have fewer trains on the Bankstown Line stop to pick up commuters.

People travelling on the Bankstown Line will need to change trains at Sydenham to get to St Peters. And those travelling from St Peters will have to catch trains on the T8 Line during peak hours to get to the central business district.