The Melbourne Metro Tunnel opens today. Here's what you need to know

Devoted Melburnians have marked a new era for the city, with hundreds lining up from the early hours to be among the first to ride through the city's long-awaited Metro Tunnel.

Limited services will run through the tunnel, giving Victorians a chance to see the most significant update to the state's rail network — at a cost of $15 billion — since the City Loop opened in 1981, with five new stations added to the network.

Fraser and his father, Craig, travelled from Camberwell to Sunbury, where the first service departed, to secure their spot at the front of the line.

Fraser said he had been waiting "years and years and years" for the tunnel's opening.

"I'm here because I'm just super excited about this whole thing. I couldn't be more excited about something," Fraser told the ABC.

"This is pretty much the most exciting thing I have had happen in the past few years."

Brendan and Will travelled from Craigieburn to board the first public service travelling through the tunnel.

Will said he was looking forward to stopping at each of the new stations, especially Arden.

"I've seen the pictures and I like the look of it, so I'm excited to see it in person," he said.

Sam, from Berwick, who caught the first train into the tunnel from Sunbury, said he was feeling "pure exhilaration" when the ABC spoke to him at Town Hall Station.

"I reckon I'm running on pure adrenaline today since I got on. The atmosphere has been amazing," he said

"I'm really excited to be able to travel on the new metro. I'll be using [it] quite a bit."

Sam said the Metro Tunnel would also help him get to Parkville for medical appointments.

"I usually head to the Peter MacCallum Cancer Centre as a cancer survivor — I have check-ups there," he said.

"Now it'll be helpful for me to be able to get there directly by train."

Premier Jacinta Allan described the opening of the project as "absolutely bloody amazing" before boarding a packed carriage on the first service.

"It's clear too that thousands of Victorians are also tremendously excited to mark this major, significant milestone that will transform Melbourne and Victoria's public transport network," she said.

Cheers rang out at various points in the journey, including when the train entered the tunnel and pulled up to the first of the new stations — Arden.

At about 11:30am Metro Tunnel services were temporarily delayed due to what a Metro Tunnel spokesperson called "unauthorised trespasses" on the network near Hawksburn Station.

Metro Trains's website said the disruption was due to a fire services request.

"The trespassers were apprehended by Metro Network Security team and referred to Victoria Police," the spokesperson said.

"The network was confirmed clear and safe, with services resuming at 11:41am."

In a statement Fire Rescue Victoria (FRV) said the government had advised the service its firefighters had "accessed an unauthorised area of the Metro Tunnel".

"This access triggered a security alarm which temporarily delayed the trains and resulted in the attendance of Metro Trains Security," the FRV spokesperson said.

"FRV is investigating the incident."

Victoria Police said no arrests were made.

On Sunday, the United Firefighters Union again expressed its concerns about the safety of the Metro Tunnel, claiming that crews responding to emergencies could not properly communicate over radio.

The state government and national rail safety regulator have strongly refuted the union's claims.

What is the Metro Tunnel?

It's a 9-kilometre train tunnel that runs under the CBD connecting the Sunbury, Cranbourne and Pakenham lines.

There are five new inner-city stations: Arden (in North Melbourne), Parkville, State Library, Town Hall and Anzac (on St Kilda Road).

Town Hall Station will connect to Flinders Street Station and State Library to Melbourne Central.

Metro Tunnel executive project director Ben Ryan said people would be "blown away" when they saw the new stations.

"These stations are at a scale that are much, much bigger than anything Melbourne has seen before," he said.

The tunnel removes three of Melbourne's busiest train lines — the Sunbury line in the north-west, and the Cranbourne and Pakenham lines in the south-east — out of the city loop and into the new tunnel.

"What that does is it enables more trains to go into the City Loop and gives access to new parts of Melbourne," Mr Ryan said.

He said changes would mean "really significant [time] savings" for travellers.

"And we really want to encourage people to check it out and see how their journeys might change … because it will really unlock better ways for people to move around our city and state," he said.

The Victorian government has said the new system would deliver an extra 1,000 weekly train services when full service begins next year.

What happens now the Metro Tunnel has opened?

Trains will begin running through the tunnel and stopping at all five new stations from Sunday morning, with limited services continuing through the tunnel during summer.

The existing timetable will remain in place across the rail network, with 240 new services added through the tunnel as part of the summer timetable.

There will be a special timetable for opening day, where trains will run every 20 minutes between Dandenong and West Footscray, with some services extended to East Pakenham in the morning and Sunbury throughout the day.

The first train into the tunnel will hit Arden Street at 10:09am, and the new stations will be open between 9am and 7pm.

For all other lines, there are no changes other than commuters being able to change to Metro Tunnel services.

The summer timetable will run until February 1, when full service will begin. All public transport will be free on weekends in December and January.

During the summer timetable, Metro Tunnel trains will only operate between West Footscray and Westall every 20 minutes from 10am to 3pm on weekdays.

On weekends, from approximately 10am to 7pm, trains through the tunnel will run about every 20 minutes between West Footscray and Westall, about every 40 minutes from East Pakenham and about every 60 minutes from Sunbury.

Cranbourne, Pakenham and Sunbury services will continue to run via the City Loop until February.

Updated train timetables can be found on the PTV Journey Planner.

What else can commuters expect?

With Melburnians set to descend underground in their droves to check out the new rail network, Mr Ryan says he's confident the Metro Tunnel's soft launch will go smoothly.

But, he says he can't rule out any teething issues.

"It's a really complex system, and it's new technology … and that means that the people operating them — the train drivers, the people running the train system, the station staff — there is a learning period as they get used to operating these new stations," he said.

"So there might be a few bumps in the road but it is absolutely safe, there is nothing to worry about."

One example of that technology is new platform screen doors at the five stations, which will display how many people are on each carriage as trains arrive.

"So if one carriage is full, you can tell in advance that that carriage is full and move down to another door and get on one with a bit more space," Mr Ryan said.