The doors on one of Sydney's new Metro driverless trains failed to open this afternoon, causing delays and huge queues, marring what the New South Wales Government hoped would be a day of celebration.

Key points: The driverless Sydney Metro train line officially opened on Sunday with commuters offered free travel to celebrate

The driverless Sydney Metro train line officially opened on Sunday with commuters offered free travel to celebrate There have been teething problems, with one train overshooting the platform causing a 15- to 20-minute delay

There have been teething problems, with one train overshooting the platform causing a 15- to 20-minute delay More than 22,000 people have worked on the project, which will extend to Bankstown by 2024

NSW Premier Gladys Berejiklian was beaming on Sunday morning at Tallawong station in Sydney's north-west as the country's first driverless train took off five years after the project began.

It is part one of the State Government's $7 billion North West Metro train network — Australia's biggest public transport project.

There were cheers as the train started at 10.40am, and cheers again when Ms Berejiklian and 300 special guests arrived at Chatswood station, in Sydney's lower north shore.

By the end of the day more than 100,000 people used the service, with thousands expected to jump on tomorrow for the line's first peak-hour run.

But the celebrations turned to stress for workers on the line about 1.00pm when one of the trains overshot the platform at Macquarie Park station.

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It was realigned and then the automatic doors failed to open, forcing workers to manually open them.

The offending train was taken off the service — and commuters were taken off the train — causing delays and massive crowds at some of the stations around Sydney.

There were lines around the block at Castle Hill station and reports of a "crowd crush" at Chatswood.

Reports have emerged of trains taking more than an hour to travel three stops. The whole journey from Tallawong to Chatswood is supposed to take 37 minutes.

A Sydney Metro spokesperson said it was putting on extra services to clear the backlog, but on Sunday afternoon there were reports of "15- to 20-minute" delays. Some commuters were reporting much longer delays.

The trains were operating normally at 3:00pm but there were still large queues at most of the stations.

The queues of people at Castle Hill station for Sydney's new Metro trains. ( ABC News )

One commuter who had three children with her said she was waiting at Rouse Hill station for "about 25 minutes".

She said it was hard to know what was going on as all the announcements were automated.

"But we got there, we survived," she said.

Speaking earlier in the day, Ms Berejikilian said the Government had "defied all the cynics" who said the project would never happen.

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"In the early days, when I spoke to local residents, the parents used to say it'd be fantastic if our kids could catch the train to university or high school," Ms Berejiklian said.

"And some of the ones who'd been here for a long time said 'I don't think I'll ever see it in my lifetime'.

"Well I'm glad to prove them wrong … it's here, and it's the start of something really special for New South Wales."

To celebrate the opening, the New South Wales Government offered free travel for all commuters on the line on Sunday, with huge crowds coming out to ride on the new trains.

The system has Australian-first technology, like screen doors attached to the platforms of all of the network's 13 stations, which open automatically after the train has stopped.

At full operation the trains, which reach speeds of up to 100km/h, will run every four minutes in each direction at peak times with the ability to run up to every two minutes in the future.

The Metro line is currently being extended from Chatswood to Bankstown, and by 2024 Sydney will have 31 Metro stations and 66 kilometres of standalone Metro railway.

But the real test of the new system will come on Monday, when commuters use it for the first time during peak-hour operations.

The trains will also stop running at 9:00pm from Wednesday to Sunday over the first few months to allow an ongoing review of the operations and early maintenance.