After the Labor Party won the 2017 state election, the Ellenbrook line began planning as part of the wider Metronet project.

The main Ellenbrook line contract was awarded to Laing O'Rourke in October 2020 and construction began in January 2022.

The 1955 Plan for the Metropolitan Region, Perth and Fremantle, also known as the Stephenson–Hepburn Report, proposed a 7+1⁄2-mile (12 km) railway line branching off the Eastern Railway (Midland line) at Bayswater, then heading north through Morley to reach Walter Road and then north-west to terminate near Wanneroo Road.

The corridor south of Reid Highway to Bassendean was not reserved due to a large number of objections,[10][11]: 1  which prompted the government to commission the North-East Corridor Transit Route Reserve Study to determine a route south of the Reid Highway/Lord Street junction.

[15][16][17] Opposition and Liberal Party leader Colin Barnett also committed to building a railway to Ellenbrook,[18][19] but in the months following the Liberal Party's election victory, the Public Transport Authority (PTA) advised Transport Minister Simon O'Brien that bus rapid transit (BRT) would be more adaptable and could follow roads through built-up areas, unlike a railway line.

In February 2013 though, Barnett reneged on that promise, saying that "when we looked at it and we took advice from the Department of Transport and others, it was clear that rail line was ahead of its time".

Estimated to cost $49 million, the new route was a 9-kilometre (6 mi) dedicated busway along Lord Street between the Ellenbrook town centre and Marshall Road.

[41][42][43] A request for tenders was released in July 2016[44] and in November 2016, CPB Contractors was selected as the preferred proponent, with the cost having risen to $55 million.

[45][46] In February 2016, Transport Minister Dean Nalder revealed the government was considering an underground rail line from the Perth central business district to Morley via Edith Cowan University in Mount Lawley, with an eventual extension to Ellenbrook.

[55] The final version of the Transport@3.5 million report, published in February 2017, said that a rail line to Ellenbrook would be needed before 2050.

[56] Meanwhile, Labor again promised to build the Ellenbrook line for $863 million as part of its revised Metronet project,[57][58] with construction beginning in 2019 and finishing in 2022.

It was also announced that the scope of an existing project to rebuild Bayswater station would be increased to add extra platforms for the Ellenbrook line.

[78] Another option considered was tunnelling the Ellenbrook line between Bayswater station and Tonkin Highway instead of building a viaduct.

Tunnelling was rejected due to the high cost, steep slopes required, the Bayswater Main Drain being in the way, and disruption to the Airport and Midland lines.

[79][80] The route through the southern and eastern parts of Whiteman was criticised by the shadow minister for transport, Libby Mettam, who said that it broke the terms of an agreement made when Lew Whiteman and other landowners sold the land to the state government which said that the land must remain public open space.

Important environmental considerations included the clearing of vegetation, impacts on Bennett Brook and nearby wetlands, and the habitat of Carter's freshwater mussels and black cockatoos.

This project's main purpose was to widen Tonkin Highway between Collier Road in Bayswater and Dunreath Drive in Redcliffe, but the associated works included building the tunnels and dive structures for the railway to enter and exit the highway at Bayswater and Ballajura, modifying drainage along the Ellenbrook line route, adding barriers between Tonkin Highway and the railway, rebuilding the Broun Avenue bridge to allow for a bus interchange at Morley station, and building railway bridges across Morley Drive.

The Tonkin Gap Alliance, consisting of BMD, Georgiou, WA Limestone, BG&E and GHD, was announced as the preferred proponent in May 2020,[97][98][99] and the $400 million contract was signed in June 2020.

From May 2021, the northbound Tonkin Highway carriageway between Railway Parade and Collier Road was shifted east to allow for the construction of the southern dive structure.

[106] From September 2021, the southbound Tonkin Highway carriageway near Marshall Road was shifted west to allow for the construction of the northern dive structure.

[136][137][138] The contract's total budget was $1.123 billion when including contingency, escalation, and ancillary costs,[139] with a further $233 million for the Ellenbrook line portion of the Bayswater station and Tonkin Gap projects.

[155][156] The May 2023 state budget revealed the cost of the Ellenbrook line had increased by $528.1 million,[123][157] bringing the total to $1.651 billion.

[172] The Ellenbrook branch uses 1,067 mm (3 ft 6 in) narrow gauge track[173] and has a maximum speed of 130 kilometres per hour (81 mph).

[8]: 20 From Bayswater, the Ellenbrook line heads north along Tonkin Highway for approximately 7 kilometres (4 mi)[8]: 29  until it reaches Ballajura.

North of Marshall Road, the Ellenbrook line enters a short tunnel and bends east under the southbound Tonkin Highway carriageway.

[8]: 20 In Bennett Springs, the Ellenbrook line bends north to travel along the western side of Drumpellier Drive, avoiding Horse Swamp, which is classed as a conservation category wetland.

[178] All stations on the Ellenbrook line branch are fully accessible[8]: 2–3  and have 150-metre-long (490 ft) island platforms, long enough for a six-car train.

Factors limiting accessibility include non-compliant ramps, a lack of tactile paving, large platform gaps, and pedestrian level crossings.

[178] By 2031, it is planned for peak frequencies to increase to six trains per hour, which will require the CBTC system to be operational and an additional siding at Daglish.

Provisions have been made for a future depot along the Ellenbrook line in Henley Brook with capacity for twelve six-car trains.