Sydney train delays are impacting some morning commuters. (ABC News: Monish Nand)
Transport for New South Wales says repairs to overhead train lines are now complete, after wiring issues caused delays on Tuesday morning.
Earlier this month, a "freak issue" with overhead wires also caused major delays on the network.
Three empty trains which came to a stop last night still have to be removed from the tracks before they can restore power.
Transport for New South Wales (TFNSW) says repairs to overhead train lines are complete, after wiring issues caused delays for the second time in almost as many weeks.
A spokesperson for TFNSW said work was undertaken to restore power and passenger services, with an update expected from Sydney Trains at 1pm.
The transport agency said it still had to move three empty trains off the tracks where they came to a stop last night.
Sydney Trains said the wiring failed overnight, and services on the T1 North Shore Line and the T9 Northern Line had been hardest hit.
"We had what's called a loss of tension in the overhead wire at Chatswood, so the line that powers the train, due to a failure in one of the counterweights," Sydney Trains CEO Matthew Longland said.
"So we have very heavy weights that hang off to the side that keep the tension, keep the wires held up. One of those counterweights failed last night.
"The counterweights are inspected regularly, so we'll obviously look at the history of this particular location, whether there was any incident there that was a problem."
Replacement buses have been made available to morning commuters facing train delays. (ABC News)
Mr Longland said crews worked through the night to fix the issue.
The city-bound service between Gordon and Chatswood on the T1 line had been replaced with buses on Tuesday morning.
TFNSW's spokesperson also said there was no estimated time when passenger services would be fully restored, but some bus replacements were still available.
