NSW TrainLink has replaced XPTs with buses on the North Coast service to Grafton after finding cracks in centre pins which link the locomotive body and wheels.

The move is designed to protect passengers with a faulty pin risking the possibility of a derailment.

"The safety of our customers and staff is our utmost priority," a TrainLink spokeswoman said.

"A small number of XPT diesel locomotives have been removed from service for testing due to cracks identified in the centre pins.

"Due to fleet availability, the Sydney to Grafton XPT has been replaced by road coaches as a result of this essential maintenance.

"These locomotives will be returned to service following the completion of all necessary and appropriate engineering assessments and actions."

The spokeswoman said the steel pins would be updated in each XPT set but Sydney to Melbourne services would continue without bus replacements.

"The XPT fleet is maintained to the highest standard and undergoes thorough inspection every 42 days," she said.

"The centre pins on the entire XPT diesel locomotive fleet will be replaced over the coming months."

The Sydney Morning Herald has reported that replacing the pins could take months due to the need for special castings and forging required for the pins.

The XPT fleet was first introduced to NSW in 1982 and is due to be replaced by 2023.