The Australian-first use of new electrical technology has allowed the new Fremantle rail line tunnel to be built just 1.2m above the existing Joondalup line tunnel.
In another project milestone, a tunnel has been widened and a new platform created at Perth station for the special football train service to the proposed Burswood stadium.
And an underground pedestrian walkway - linking all platforms with the Mandurah line platform and the heart of the city in Murray Street - has been dug.
The new technology, known as solid rail conductors, is being used in the 600m Fremantle rail tunnel instead of the overhead lines that power the rest of Perth's suburban train network.
The conductors are fixed to the tunnel roof and remove the need for masts, bracing arms or hanging wires.
They have only previously been used on the Naples Metro in Italy, the Vienna Airport express in Austria and the Berlin rail station in Germany.
The Perth project's interface manager, Nick Stephens, said the technology meant a smaller clearance between the tunnel roof and the top of the trains.
"Without this technology the Fremantle and Joondalup tunnels would be much closer together," Mr Stephens said. "Initial planning had the gap between the two tunnels at only 70cm."
Project director John Anderson said building the Fremantle line above the Joondalup line - for a distance of about 50m - posed many challenges and there was potential for the Joondalup tunnel to move.
With the water table less than 1.5m below ground level, more than 300 wells had been installed to pump away water.
About 100 monitors were placed in the Joondalup line tunnel to indicate any potential movement.
To connect the Fremantle line tunnel to the entire network will require the biggest shutdowns in Transperth's history - one from July 12 to 17 and the other from July 31 to August 4.
During the first shutdown, the Midland and Armadale line trains will terminate at McIver on weekdays and Claisebrook on the weekend. Joondalup and Mandurah trains will terminate at Leederville and Esplanade respectively, while the entire Fremantle line will be closed.
The Midland, Thornlie and Armadale lines will not be affected during the second shutdown.
The new platform 9, on the Roe Street side of Perth station, will be used to take commuters to the new Burswood stadium but could also be used for other special event venues, including the Perth Royal Show.