Train commuters coming from the northern side of Sydney going into the central district and vice versa have been advised to squeeze in extended time in their travel itineraries after the Newcastle and Central Coast Lines got partially closed due to "urgent overhead wiring repairs."

Transportation authorities meantime have deployed buses between Hornsby and Epping to address the commuter backlog.

"Passengers that normally travel on the Northern Line are advised to catch a replacement bus to Epping and then continue on a train towards the city or catch a train to Hornsby and travel via the North Shore Line," a spokeswoman from the Transport Management Centre (TMC) said in a statement on Friday morning.

"Urgent overhead wiring repairs" are currently taking place at Normanhurst when a train driver reported losing power at about 5.30am (AEDT). Although he was still able to restart the train, and was able to still travel up until Epping, the train however tore five kilometres of overhead wiring.

"We'll be running trains to the north, there will be some delays because of congestion and anyone going to the Central Coast or Newcastle, listen to announcements, look at websites because your trains will be late, they'll be leaving out of their normal locations but they may be late or out of timetable order," Tony Eid from Railcorp said.

About 100 people were evacuated from the train. No injuries were reported.

TMC announced trains from Newcastle and the Central Coast will make additional stops at Chatswood and then additional stops at North Ryde, Macquarie Park and Macquarie University.

"Passengers on the North Shore line may experience an increase in journey time, however a frequent train service is still operating," the TMC spokeswoman said.

TMC did not give any assurance as to when normal services will resume.

Passengers are advised to check indicator boards, listen to announcements and allow extra travel time.