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As most parents will attest, finding a good child care spot in the city is hard enough without getting it pulled from under you - literally.

One of the biggest logistical challenges of the $360 million City Link project to sink underground the rail line between the CBD and Northbridge, was temporarily closing a playground of the child care centre at Perth's rail station to allow tunnelling under it.

Now replaced, and filled with happy kids, the shuffling of the centre allowed a new platform to be built underneath that will eventually service the Burswood football stadium.

The complex project passed a key milestone last week with its biggest concrete pour.

It places the venture - an alliance between lead contractor John Holland and the Public Transport Authority - ahead of schedule to begin testing the first trains through the 600 metre tunnel.

They will now run underground as early as July, up to six months sooner than expected.

That timetable win has sprung its own logistical challenge, with the PTA likely to bring forward work on a temporary terminal to handle buses. That's to accommodate the demolition of the existing Wellington Street terminus sometime in the next year, subject to a yet-to-be awarded contract.

Adam Harry, John Holland's general manager, western region, said careful planning and a "construction sweet spot", where a cooling in the mining sector eased the price of wages and materials in the rest of the State, had helped the project to record something of a rarity in WA: it is on time and on budget.

Having recently passed its riskiest stage - the crossing within 1200 millimetres of the existing Joondalup to Mandurah rail tunnel - touch wood, the project will finish early next year, rather than in June as first planned.

That close shave with the existing train tunnel network was improved by 600mm with the use of slim-line electrical conductors, previously used only in Europe.

It was a reminder of the complex challenges faced by the project, which are rare in land-rich WA but common in tight urban spaces, such as Hong Kong.

"It's a unique site and we're using techniques that are rarely used collectively on projects in Western Australia, but are often seen on projects in high density cities such as Hong Kong or Singapore," the construction alliance's manager John Anderson said. "It's a great experience for our engineers and comparable on an international stage."

Assuming it stays on track, the venture should prove John Holland's doubters wrong after it bid aggressively to win the project.

About $40 million cheaper than the next bid, Mr Adams said John Holland had learned the lessons from its troubled Airport Link project in Brisbane to use a tried-and-tested tunnelling technique in WA to ensure costs were kept low.

It also paved the way for developing at least five skyscrapers on land "freed up" by the tunnel.

Ironically, it could have been so much cheaper for WA taxpayers if the State Government had stayed the course in 1970, when the project was first ticked off and before the oil crisis derailed it.

A news article from the era captures the excitement of the pending tunnel venture, talked about since Federation. "The Western Australian Government plans to have the railway lowered through central Perth by July, 1975, using its own financial resources," the article said.

"A new underground city station is part of the plan, which has also been made possible by the conversion of suburban motive power from steam to diesel and diesel-electric." Costed at $10.25 million at the time, that would amount to $105.9 million in today's dollars.

We're using techniques that are rarely used collectively on projects in Western Australia. "Perth City Link Rail Alliance manager John Anderson