The Thornlie line has been suspended since 20 November 2023 due to construction work; it is planned to reopen in mid-2025.

The main construction contract was awarded to Barclay Mowlem in 2004 and the Thornlie line opened on 7 August 2005.

The main construction contract was awarded to a joint venture between CPB Contractors and Downer in December 2019 at a cost of A$716 million.

Since 20 November 2023, the Thornlie line has been temporarily closed to allow for works on the Victoria Park-Canning Level Crossing Removal Project and the Thornlie–Cockburn Link.

The South West Metropolitan Railway would have diverged from the Armadale line at Kenwick, entering a tunnel to pass under the Perth-bound Armadale line track, Albany Highway, Roe Highway, and the freight railway.

[2] The 200-metre-long (660 ft) Glen Iris tunnel was also constructed as part of an extension of the Kwinana Freeway in 2000.

[11] The second stage contract for the Kenwick tunnel was awarded to John Holland Group in March 2003 at a cost of A$14.225 million.

The second stage consisted of 380 metres (1,250 ft) of tunnel; the realignment of 1.4 kilometres (0.87 mi) of the Kwinana freight railway, which was leased by Australian Railroad Group at the time; and the construction of noise walls.

[14] Construction was also disrupted by a solidarity strike in January 2005 in relation to pay conditions for workers on the Mandurah line.

This involved constructing Thornlie station, 2.9 kilometres (1.8 mi) of single track railway, a rail bridge across the Canning River and a road bridge for Spencer Road across the Thornlie line and freight railway.

[19] Barclay Mowlem was announced as the preferred tenderer in February 2004 and was awarded the contract by April 2004.

[35][36] Ahead of the 2017 state election, both major parties committed to extending the Thornlie line to Cockburn Central station.

Infrastructure Australia recommended that the contract for the Thornlie–Cockburn Link be combined with the contract for the Yanchep Rail Extension of the Joondalup line to save costs during procurement,[47][48][49] although local contractors expressed concerns that the two projects had different requirements, were in different parts of Perth, and that combining the projects would price out smaller contractors.

[56][57] The project was to involve the construction of two new stations: Nicholson Road and Ranford Road stations, each with two 150-metre (490 ft) long island platforms, full disabled access, a bus interchange for feeder buses and a park and ride.

14.5 kilometres (9.0 mi) of new dual-track railway was to be constructed between Thornlie and Cockburn Central, using the pre-existing Glen Iris tunnel to enter the median of the Kwinana Freeway.

3 kilometres (1.9 mi) of single track railway between Beckenham junction and Thornlie station was to be duplicated, including constructing a new bridge across the Canning River.

To make way for the Thornlie line, 11 kilometres (6.8 mi) of the freight railway was to be relocated slightly north.

[58]: 12 Thornlie station was to have its platform's length extended to 150 metres (490 ft) and be converted to allow for through trains, including the construction of an overpass.

[58]: 6 A request for proposal was released in September 2018 for the design and construct contract for the Thornlie–Cockburn Link and the Yanchep Rail Extension.

[62][63][64] In November 2019, NEWest Alliance was announced as the preferred proponent to design and build the Thornlie–Cockburn Link and the Yanchep Rail Extension.

The delay was blamed on negotiations with BP for relocating the Kewdale White Oil Line and with Arc Infrastructure, the new leaseholder of the Kwinana freight railway.

The Nicholson Road bridge, which was constructed between December 2016 and May 2018 to replace a level crossing, did not have to be rebuilt as it was designed with the Thornlie–Cockburn Link in mind.

[75][76][77] The Karel Avenue bridge was originally constructed in 2005 and 2006 to replace a level crossing as part of the extension of Roe Highway from South Street to the Kwinana Freeway.

[95] The September 2021 state budget revealed that the opening of the Thornlie–Cockburn Link would be delayed by a year to late 2024 due to a skills shortage and to ease pressure on Western Australia's construction industry.

[105][106] The same section of the Mandurah line was shut down again from 26 December 2022 to 3 January 2023, with the exception of New Year's Eve.

[109] An open trench pipeline was originally considered during planning, but the desire to limit disruption to the freight railway meant that predominantly horizontal directional drilling was the final option chosen.

Works would be conducted for the Thornlie–Cockburn Link during the shut down as well, such as duplication of the tracks between Beckenham junction and Thornlie station.

[117] This was completed by October 2023, with freight trains running along the new alignment at that point, allowing for tracklaying for the Thornlie line to commence.

[123] The Thornlie branch uses 1,067 mm (3 ft 6 in) narrow gauge track[124] and has a maximum speed of 110 kilometres per hour (68 mph).

[129] Thornlie line peak frequencies are forecast to remain the same until at least 2031, with capacity increases to be provided for by longer trains enabled by platform lengthening.