The lead car of the empty two-car passenger train came off the tracks at Port Kembla just before 5.30am on Tuesday.

It was leaving the stabling yards and heading to Wollongong to begin passenger services at the time.

While the train wasn't on the main line when it derailed, a TrainLink NSW spokesman said it was close enough to the passenger line to stop trains for 15 hours.

Buses replaced trains between Wollongong and Port Kembla in the morning and evening peak and until 9pm when the train was placed back on the tracks.

“Once specialist crews arrived on site and the overhead power was isolated, the work involved excavating around the train and using jacks to lift the carriage back onto the tracks,” a TrainLink NSW spokesman said.

“Once this was completed the train was able to operate away from the site for repairs with special procedures in place.”