Vandals have broken into a railway station in Melbourne's north-east, started a train and then derailed it, "destroying everything in its path" after seemingly obtaining a key on the black market.

Key points: Vandals cause $3 million damage after crashing train at Hustbridge

Vandals cause $3 million damage after crashing train at Hustbridge Metro says key was needed, possibly obtained on black market

Metro says key was needed, possibly obtained on black market Station night watchman and cleaner on site but not injured

Station night watchman and cleaner on site but not injured Clean up could take days, possibly weeks

Victoria Police believe the offenders broke into the train at the Hurstbridge Railway Station about 1:50am.

They managed to start the train and drive it forward, before running into carriages, fencing and security boxes.

Metro Trains chief executive Andrew Lezala said the incident had caused about $3 million in damage and a key would have been needed to start the train.

"Somebody has gained access to one of our trains and managed to motor it up," he said.

"The train has been derailed by the derailing device, which is there to stop trains getting onto the main line when they shouldn't be there.

"So it's done its job, but [the act has] caused extensive damage."

Keys for Metro trains are not kept in the carriage but are looked after by drivers and key staff.

Loading

The train was an X-Trapolis model, which operates on a universal key system - meaning any train of that model can be started with the same key.

Mr Lezala said they were "occasionally stolen" and those involved may have obtained one through the black market.

"This is a severe act of vandalism," he said.

"I am very concerned that somebody has been able to do this and we will work with the police to apprehend these people, and I hope they get very severely dealt with.

"This is a situation that we just cannot tolerate."

Sergeant Mark Chetcuti from Victoria Police said police were investigating the circumstances around the incident.

"They've used a rock to force the lever and put the train in motion, causing that to drive for 40 to 50 metres and basically just destroying everything in its path," he said.

A security guard and a cleaner were at the station at the time, but no one was injured.

"There was a cleaner and a night watchman who obviously saw the activities unfold, and just had to move away and put themselves in a safe position, there was not too much they could do," Sergeant Chetcuti said.

"They got onto triple-0 pretty quick and the response was pretty prompt."

Victoria Police's transit criminal investigation unit will take over the investigation.

Hurstbridge services to be disrupted for days

Police are have urged any witnesses to come forward, and are looking at CCTV.

Services on the Hurstbridge line between Hurstbridge and Diamond Creek have been suspended while the clean up continues.

Earlier Leah Waymark from Metro said buses were replacing trains on the affected part of the line and major delays could be expected.

"It's going to take us a long time to recover this, we're not sure exactly how long as we're still inspecting the extent of the damage here," she said.

"My read is that it's going to take us days, certainly not two weeks at this stage, but it will take us days, and in the meantime we'll be running a full service for our Metro customers, albeit buses replacing trains between those three stations."

She said a crane would be needed to repair the damage.

"There's quite extensive works, there's damage to the overhead, there's damage to the trains - three of the four cars are fully off the tracks here, so the extent of the damage is quite extreme," she said.

Customers have been advised to check the Metro Trains website or the MetroNotify phone app for the latest information.

Train security needs to be increased, union says

The state secretary of the Rail Tram and Bus Union, Luba Grigorovitch, said the incident could have been prevented if security had been upgraded.

"It's fortunate that no-one was severely injured in this instance, but the reality is there was a security guard present when this train was hijacked," she said.

"We've been calling on Metro for many, many months now to make sure that they up security at sidings and yards."

Metro has privately employed security officers but only 15 specialist surveillance officers.

"This is totally unacceptable," she said.

"Metro trains need to lift the amount of network security officers from 15 to at least 50 at an absolute minimum.

"The Rail Tram and Bus Union has raised concerns about trespassers and vandals on a number of occasions, they've fallen on deaf ears.

"I'm hoping this time that both Metro and the Government listen."

Ms Grigrovitch said you can buy Metro Trains uniforms online which "poses a huge risk" to our members.

"If you've a vandal putting on Metro clothing and pretending to be one of the staff, well that's just not on," she said.

Metro is looking at changing the keys and will increase security at its 60 depots across the network.