Commuters on Sydney's rail network have been experiencing delays of up to an hour because of overhead wiring and signal problems.
The rail system succumbed as Sydney experienced its hottest day on record, with the mercury hitting 45.8C just before 3pm (AEDT).
Passengers took to the social media network Twitter to complain about sweltering on rail stations or being stuck in train carriages without air conditioning.
Train tickets can be used on buses and ferries, the Transport Management Centre says.
However, services have resumed between Campbelltown and Moss Vale on the Southern Highlands Line after being suspended earlier on Friday because of a bushfire at Glenlee.
Trains to the city on the Airport East Hills Line are also now stopping at Kingsgrove, Bexley North and Bardwell Park.
Buses continue to replace trains between Wyong and Fassifern on the Newcastle Central Coast Line while urgent overhead wiring repairs are carried out at Dora Creek.
Buses are also still replacing trains between Maitland, Scone and Dungog on the Hunter Line due to mechanical problems with a train.