WA's premier and treasurer were among the first passengers to ride the 15-kilometre railway extension covering three new train stations in Perth's north — Alkimos, Eglinton and Yanchep — on Sunday.

The train was packed to the brim, with more than 500 passengers on board to the city's most northern suburbs for the first time.

The crowd clapped as the WA-assembled train departed the Alkimos station.

Young families, older patrons and train enthusiasts alike packed out each carriage, some filming the journey out the window to mark the occasion.

At each of the new stops the hordes of people spilled out into the double-storey stations, kitted out with art installations.

Passengers travelled for free on Sunday, but the ride did not come cheaply for taxpayers. 

The project's price tag has doubled since it was first announced in 2017, costing the government more than $1 billion and opening almost two years later than promised.

'Mission accomplished'

Metronet will add 23 new stations to Perth's railway network, with the state government estimating it will turn about 8,000 hectares of land into more desirable places to live.

Despite delays and price blow-outs, Premier Roger Cook declared the opening of the Yanchep extension a success in connecting the outer suburbs.

"Thanks to the hard work of 1,300 workers, we are finally happy to say, 'mission accomplished'," Mr Cook said.

"As a dad from Kwinana, I understand just how important this sort of transport infrastructure is.

Transport Minister and Treasurer Rita Saffioti said it was an exciting day for the northern suburbs.

"It's about connecting communities, it's about ensuring that families that are moving into the outer suburbs have excellent public transport from day one," Ms Saffioti said.

"This is a spectacular project that will serve generations to come.

"Alkimos and Eglinton has a lot of land adjacent to these stations to make sure that we can integrate new housing and commercial developments early into the station precinct."

Ms Saffioti defended monetary blowouts, citing significant cost increases for importing reinforced steel, diesel and concrete during the project's timeline, as well as the COVID-19 pandemic.

"As we know, costs have increased across the construction sector in WA and nationally, and also we've had a significant contribution from the federal government," Ms Saffioti said.

"The proudest day for me is when you meet people who will rely on public transport to connect to jobs, to connect to education opportunities."

Locals welcome line

Local residents have welcomed the extension in the hope it will bring more business to the coastal suburbs.

Jacinta and Daniel have lived in Yanchep for the past six years and said the train station was all part of the area's rapid growth.

"We've obviously seen a lot of change just in those years. I think that it's definitely become a lot more popular," Jacinta said.

"I think people are surprised that it's not as far north as it was previously thought."

Annie works at the local barber shop and said easier access to the northern suburbs had been a long time coming.

"I think for a long time Yanchep wasn't accessible, especially by public transport, so you either had to live out here or be able to get here on your own," she said.

"I think with that rail opening up it's going to give us a lot more exposure.

"Yanchep was like a destination where you kind of go camping and you take a while so now that it's built up and we're getting a rail in I think it's going to do good things and bring good people out here as well."

Gap in plan

The independent climate researcher Climate Council released data this month which stated 60 per cent of people in Perth don't have access to decent public transport.

The report rated Perth the second-worst capital city in the country, leaving more than a million people without access to frequent, all-day public transport.

The group found the minimum level of service required to get people out of their cars was access every 15 minutes, between 7am and 7pm and within 800 metres of their homes.

The proximity of the new stations to homes was a gap in the state government's plan according to Professor of Sustainability at Curtin University, Peter Newman.

"It's a bit like going through country towns, there's not much else around them," he said.

Professor Newman said trains were the best way to bring a city together, but the newly built suburbs of Alkimos and Yanchep were too spread out to reap the benefits of the new railway.

"You've got to build around the stations to make that work," he said.

"There's some access, but they're scattered and not dense enough so you can't actually make it all work.