The state's decades-old XPT trains, which run through the Hunter to northern NSW and Brisbane, have been plunged into a safety crisis after the discovery of cracks in a critical component of the fleet.
Sydney Trains, which maintains the NSW TrainLink-operated trains, has been forced to begin replacing centre pins on the entire fleet of XPT locomotives.
The centre pins connect the body of a locomotive to its driving wheels. Failure of the component while a train is in operation risks causing a derailment.
The issue has forced four XPT diesel locomotives to be removed from operation and some train services to be cancelled, including the daily Sydney to Grafton service which has been replaced by buses in both directions indefinitely.
"These locomotives will be returned to service following the completion of all necessary and appropriate engineering assessments and actions," Sydney Trains said in a statement.
"The safety of our customers and staff is our utmost priority."
The replacement of the pins, if necessary, is understood to be a lengthy process due to the complexity involved in making castings and forging the steel pins.
Passenger carriages in the XPT fleet are not subject to the same loads as the diesel locomotives but Sydney Trains will inspect a number of the centre pins on carriages to provide extra safety assurances to the public.
The operator, NSW TrainLink, could not say on Friday how long the Grafton XPT train, which stops at Fassifern, Broadmeadow, Maitland, Dungog and stations all along the North Coast line, would be out of action.
The daily Sydney to Brisbane and Sydney to Casino XPT trains continue to operate and service those stops.
"The XPT fleet is maintained to the highest standard and undergoes thorough inspection every 42 days," a NSW TrainLink spokesperson said.
"The centre pins on the entire XPT diesel locomotive fleet will be replaced over the coming months."
The first XPTs began running almost four decades ago, and concerns have been raised in recent years about their aging condition.
The NSW government ordered a new $1.26 billion fleet of regional trains, which will gradually replace the XPT, Endeavour and Xplorer trains, in February.
The trains, which will be built in Spain, are scheduled to be introduced from 2023.
A new maintenance facility is being built at Dubbo as part of the contract.