Regular services started running through Melbourne's Metro Tunnel on Sunday morning, stopping at five new underground stations that feature platform screen doors and shops.
The maiden public journey left Sunbury Station in the city's outer northwest at 9.28am and reached Arden Station at 10.09am, before moving on to Parkville, State Library, Town Hall and Anzac stations.
The 9km twin tunnels will host services alongside the existing City Loop timetable for the Sunbury, Cranbourne and Pakenham lines over the next two months.
Mark Steffens, 18, was among those lining up to board the first train after flying in especially from Perth on a plane that arrived at 1am on Sunday.
The 18-year-old, who runs YouTube channel Transport of Perth, will ride all the way through the tunnel before doubling back to check out each station.
"It's so good how it's going to interconnect the lines here and reduce how busy the City Loop is because it's already over capacity," he said.
Samual Anderson-Young, from near Berry in NSW, said he had been following the project for some time and happened to be in Melbourne for opening day.
"It definitely will be a good thing for Melbourne," the 22-year-old train enthusiast said.
Julius Espinosa got up at 5am to train into the city from Laverton and out to Sunbury to catch the first train.
"It's wasn't easy but it's worth it," the 17-year-old said.
"I'm looking forward to the timetable changes."
In the "summer start" period, Metro Tunnel services will run every 20 minutes from 10am to 3pm between Westall and West Footscray on weekdays and every 20 minutes from 10am to 7pm on weekends.
Passengers don't need a myki card with travel free across the network each weekend through to February 1.
At that point, three lines will be ripped out of the City Loop and the tunnel's "turn-up-and-go" timetable will operate, with trains arriving every few minutes during peak periods.
Lachlan Tansey, 16, took the train from St Albans and was particular excited to check out Town Hall Station.
It will transform the way he commutes to and from the city.
"It's a good idea for them to have this period where you can travel through ... to get used to how their journey's going to change," Mr Tansey said.
Melbourne's second underground rail line has been a long time coming.
A Melbourne Metro rail tunnel was part of the Brumby government's 2008 Victorian Transport Plan, undergoing scope and route changes before the final design was settled in its 2016 business case.
Premier Jacinta Allan, who has been part of every stage of the project, said she was immensely proud.
She was joined on the train by a raft of Labor MPs, media and her family, with applause erupting as it entered the tunnel.
"It's absolutely bloody amazing to see this day come," the premier said.
Several cost escalations have pushed the price tag for taxpayers to $13.48 billion, up from its original estimate of $10.9 billion.
After factoring in extra payments from builders, the total project bill is above $15 billion.
Public Transport Users Association spokesman Daniel Bowen acknowledged there had been diversions along the way but argued the tunnel would bring benefits worthy of its huge cost.
"It's the sort investment you have to make in a city to keep things moving and make sure that people can get around," he said.
"The City Loop, back in the day, was incredibly expensive as well but I don't think anyone doubts the benefits that it's brought to Melbourne."
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