A BROKEN axle is believed to have caused the partial derailment of a coal train that damaged about five kilometres of rail track west of Muswellbrook.
The accident on Wednesday afternoon was the fourth involving a Pacific National coal train in the Hunter since late June but a senior rail investigator said he saw no evidence "at this stage" of any systemic problems with the company's operations.
Pacific National hauls more than two-thirds of the Hunter's coal.
The fully laden train was on its way to Newcastle from Xstrata's Ulan mine, near Mudgee, when the derailment happened about 4pm.
Authorities confirmed early reports of a "foreign object" on the tracks but later said a broken axle was a major factor in the derailment.
It was unclear last night whether a broken axle caused the derailment by damaging the tracks or whether an "object" damaged the axle. Pacific National said one wagon was derailed, but remained upright.
Investigators went to the site on Wednesday night and repair crews were on the job yesterday, lifting rail cars off the single track to allow it to be replaced and repaired.
The Australian Rail Track Corporation, which operates the tracks, said it would take until Sunday evening to repair the damage and reopen the line to Mudgee.
About five kilometres of track was damaged and some sections would need news rails and sleepers.
The speed limit on the line involved was 60km/h.
The Hunter Valley Coal Chain Co-ordinator, which oversees the movement of coal to the port, said the main line from Narrabri to Newcastle was open.
But the Mudgee line was closed, stopping coal movements from the Ulan, Moolarben, Wilpinjong and Mangoola mines.
Office of Transport Safety Investigations chief investigator Paul O'Sullivan said it was in the coal industry's commercial interest to minimise accidents but "given the volumes that are carried then mishaps will still occasionally occur".
Pacific National's run of accidents began on June 28 when one train hit another that was unloading coal on Kooragang Island.
A fully laden coal train, stalled on the Nundah bank, north-west of Singleton, was hit by the two locomotives sent to rescue it on Wednesday, August 22, and another shunting collision happened at the Carrington coal terminal the following day.