The Mandurah line is a commuter railway and service on the Transperth network in Western Australia that runs from Perth south to the state's second largest city Mandurah. The service is operated by Transperth Train Operations, a division of the Public Transport Authority. The line is 70.1 kilometres (43.6 mi) long and has 12 stations. At its northern end, the line begins as a continuation of the Joondalup line at Perth Underground, and ends as a continuation of the Joondalup line at Elizabeth Quay. The first 1.3 kilometres (0.81 mi) of the line is underground, passing under the Perth central business district. The line surfaces and enters the median of the Kwinana Freeway just north of the Swan River. It continues south down the freeway's median for 30 kilometres (19 mi), before veering south-west towards Rockingham. The final stretch of the line goes south from Rockingham to Mandurah.

Perth Underground railway station is a railway station within the Perth central business district in Western Australia. It is adjacent to the above-ground Perth railway station and is sometimes considered part of that station. Perth Underground station is served by Joondalup line services heading north and Mandurah line services heading south. It was built as part of the construction for the Mandurah line and was known as William Street station during construction due to its location on William Street. Perth Underground station consists of an island platform and a concourse below ground. There are five entrances to the station: from Murray Street Mall, Raine Square, 140 William Street, underneath the Horseshoe Bridge, and from Perth station.