A rail employee may have earned the title of Sydney’s most essential worker after their sick day delayed hundreds of thousands of commuters.

The signaller who helped operate trains in western Sydney called in sick on Thursday.

Unable to find someone to cover the desk, trains on the T1 Western and T5 Cumberland lines ground to a halt for more than an hour.

But this seemingly short service break was the catalyst for a night of painful delays.

Almost 500,000 Sydneysiders endured delays and cancellations, and buses replaced regular rail services until 1am Friday.

Acting NSW Transport secretary Howard Collins explained the ripple effects stemmed from a shortage of transport signallers and rostering issues.

“Signallers, a bit like air traffic controllers, are specialists,” he said on Friday.

“It takes over a year to train these people. It’s not as if we could just put anyone on the desk and operate what are vital safety control systems.”

The late notice, other sicknesses and colleagues being on leave for school holidays meant normal contingencies were not available and the desk was closed for almost 90 minutes.

While Sydney Trains has amended rostering to avoid a similar incident, Mr Collins said there were times when “things happen in alignment and we’ve got to be prepared for that”.

Many of NSW’s 314 signallers are based in the regions and others were not trained to handle the complex role vacated by the sick worker.

According to Mr Collins, the dearth of trained signallers resulted from a belief they were unnecessary.

“There was a period of time when the anticipation of technology coming on board meant there was a theory that we did not need to recruit more signallers,” he said.

But project delays have prompted Sydney Trains to recruit 30 signallers this year, with plans to hire another 30 in 2024.

The trainees will spend a year building up their skills before they can take on the job.

Cabinet minister John Graham apologised for the disruptions but stopped short of heeding the opposition’s call for a fare-free day.

Labor had called for the same measure when train disruptions paralysed the network under the Perrottet administration.

The delays came as the harbour city endures a $97 million train maintenance blitz that will derail commuters’ weekend plans for the next year.

Nearly 6000 people have worked to repair 350 kilometres of track in the past month, the transport department said.

– AAP