The reason for a broken rail line which caused six hours of disruption on the Illawarra line on Wednesday is still unknown.
Thousands of commuters were delayed from 6am after the problem with the rail line developed between Oatley station and the Georges River Bridge.
There were further delays during the afternoon peak, caused by two people trespassing on to the tracks at Redfern.
During the morning problems, a station announcement at Sutherland said service disruption was due to a break in the rail line, caused by the cold weather.
Rail breaks occur for a variety of reasons, including during cold weather as steel expands and contracts with changes in temperature.
Sydney Rail figures show, in 2016 /17, there were 11 broken rail incidents on the network
A Sydney Trains spokesman could not confirm the cause of the break on Wednesday.
“We are investigating the cause of the broken rail incident at Oatley including whether it was weather related.” he said.
“To prevent these incidents from occurring, Sydney Train uses an ultrasonic vehicle to x-ray the network to detect potential breaks.
“We are also investigating new methods and techniques to prevent these events from occurring.”