Train drivers were stranded in Melbourne’s new Metro Tunnel for almost five hours when a systems fault brought trains to a standstill during test operations last week.

The incident comes just months before the $15 billion underground train line opens to the public, with the Allan government promising it will be taking passengers by the end of this year.

An email sent to Metro Trains staff on Tuesday morning sought to reassure drivers about the fault, which occurred last Monday while operating test trains from West Footscray to Hawksburn through the tunnel.

“There was an incident that involved all trains being brought to a stand in the tunnel and then moved to platforms to hold,” says the email seen by The Age.

“Some drivers had to remain in their cab at a platform for approximately four to five hours without relief.”

The email says it took Metro longer than expected to identify the cause of the fault. Relieving drivers was further complicated by the need to get a replacement driver to a platform at one of the two CBD stations still under construction.

Metro did not disclose to staff the cause of the fault but said it had completed a full investigation and put “mitigations” in place to ensure the same issue does not occur when the Metro Tunnel opens to passengers later this year.

A spokesperson for the Victorian Infrastructure Delivery Authority said the purpose of the testing phase was to identify and rectify issues before the tunnel opens to passengers. The issue was rectified and test trains resumed the same day, they said.

“The project is currently at its most complex phase – as we continue testing advanced technology that has never been used on the Victorian rail network,” the spokesperson said.

The Metro Tunnel project consists of a nine-kilometre underground tunnel under the CBD between Kensington and South Yarra. It will connect the Sunbury and Dandenong lines via five new inner-city stations.

The project is in a period of intense testing and trial operations before passenger services begin.

The Age has previously revealed that the project is working towards a “soft opening” in November, with limited services at first, before implementing a full-service timetable in February next year.

Trains are controlled automatically by a new high-capacity signalling system in the tunnel.

One source familiar with the incident, but unable to speak publicly, said at least five trains were in the tunnel during the incident. The automated signalling system applied emergency brakes during the fault, bringing the trains to a sudden halt without input from the drivers, they said.

Up to 12 trains are being used at once during the testing phase, and they are operating as frequently as one every three minutes in each direction.

Last year, the NSW government was forced to delay the opening of Sydney’s new Chatswood to Sydenham metro by two weeks after failing to gain approval from the Office of the National Rail Safety Regulator (ONRSR) in time.

ONRSR confirmed to The Age that the Metro Tunnel project was “progressively providing the evidence” from its testing programs for the regulator’s assessment.

“The assessment verifies that all risks to safety have been identified and that the appropriate systems and processes are in place to manage them,” an ONRSR spokesperson said.

“ONRSR will assess the application as quickly as possible (once all the evidence supporting the safety case is received) noting that it requires sufficient time to undertake that assessment.”

NSW Premier Chris Minns later apologised for announcing an opening date for the Sydney Metro before it received final safety approvals.

Construction of the Metro Tunnel began in early 2017.

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