Preparations are underway for the Mandurah line shutdown, between Aubin Grove to Elizabeth Quay, from December 26.
The shutdowns will be part of construction on the Thornlie-Cockburn Link.
Some minor changes along the Kwinana Freeway southbound between Berrigan Drive and Armadale Road are also required to facilitate the shutdown.
From September 27 to early 2022, speeds will be reduced and some traffic will be shifted to a new road built alongside the freeway, to allow for works in the rail reserve.
The Berrigan Drive entry ramp to the freeway will also remain closed. Drivers should proceed with caution and adhere to the posted speed limits and directional signage in place.
The Mandurah Line - which services 60,000 journeys-a-day - will close from Aubin Grove to Elizabeth Quay stations from December 26 for a minimum of 20 days.
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The shutdown is required to shift the Mandurah Line tracks further apart to enable the new Thornlie-Cockburn Link tracks to be built in between them.
In June, Transperth conducted a survey of 4,500 Mandurah Line passengers to understand what travel changes commuters may make during the shutdown.
The feedback is helping shape public transport replacement services, with detailed information to be released in November, according to a state government statement.
The Thornlie-Cockburn Link will be Perth's first east-west cross line connection, linking the Mandurah and Armadale lines and making travel through the south-eastern suburbs quicker.
Transport Minister Rita Saffioti apologised for the inconvenience of the train shutdown and freeway changes.
"The changes to the Kwinana Freeway southbound include the speed limits being reduced between Berrigan Drive and Armadale Road, along with some lane changes. We want to give people as much notice as possible so they can plan ahead and be prepared for disruptions.
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"We have conducted a passenger survey and are talking to affected stakeholders, including Fiona Stanley Hospital, to gather their feedback to inform train replacement service planning.
"The AFL Grand Final coming up this weekend highlights just how important the Thornlie-Cockburn Link project is, providing residents in the south-west corridor a direct link to the stadium."