Why a telco outage has stopped some trains but not others

Passengers in the NSW Hunter region have been diverted to replacement buses, due to the lasting impacts of the Telstra outage. (ABC News)

A widespread Telstra outage brought some regional train services to a halt due to their reliance on 4G capabilities of the freight rail operator ARTC.

The Telstra outage that left millions of mobile phone users cursing and retailers relying on cash rather than payment systems has also stranded some train passengers.

Those on board services in the Sydney metropolitan area might not have noticed any change to timetables, but commuters in regional NSW have been left wondering how a telco's troubles could literally stop a train.

RMIT University associate professor Mark Gregory, who researches telecommunication networks, systems and services, said cellular networks are used to link trains with control centres to ensure they run safely.

He said when the 4G network went down, safe operations were compromised and reliant train services had to be suspended.

"So those systems in the data centre will start saying, 'hey, I'm expecting a message or didn't get a message or messages are not getting through'," Dr Gregory said.

Ahead of the 3G closure in late 2024, the Australian Rail Track Corporation (ARTC) upgraded technology on its National Train Communications System with 4G capability.

Yesterday's outage is, as of Thursday morning, still impacting communications across its 9,600-kilometre network, which includes passenger services on the Southern Highlands Line and the Hunter Line.

Sydney Metropolitan services, which do not rely on ARTC's 4G network, were not affected.

ARTC issued a statement focused on safety during the outage.

"As a safety precaution, ARTC has implemented established emergency protocols and safely brought rail services on the network to a stand while the telecommunications outage is resolved," a spokesperson said.

Telstra's chief financial officer Michael Ackland said Wednesday's outage was caused by malfunctioning "nodes" that manage time synchronisation within some of the network data centres.

But telecommunications consultant Paul Budde, who has spent decades living in the Hunter region, said there were shortfalls in the way networks were configured in Australia.

Paul Budde says there are shortfalls in the way networks are configured in Australia. (Supplied: Paul Budde)

He said a single fault or synchronisation issue could cause widespread impacts due to the way networks operate in Australia

"We need to address the resilience of the networks, not just the reliability of the network," Mr Budde