Railway operator Metro Trains has cancelled dozens of services as Melbourne braces for extreme heat on Thursday.
Temperatures are forecast to reach 40 degrees Celsius on Thursday, which are above the 38C threshold for speed restrictions.
As a result, Metro has cancelled 50 services tomorrow and on Friday to accommodate the trains running at reduced speeds.
Spokesman Daniel Hoare says the cancellations are necessary to ensure passenger safety.
"This happens everywhere, when you're running an above ground railway line like we are, where you get extreme temperatures, heat or cold," he said.
"The tracks will expand if they are made of steel.
"(So) in terms of safety, (we) slow these trains down and make sure they're running at a speed that is safe."
There will be cancellations on Friday as well, despite the fact it will not be as hot.
Mr Hoare says the network needs time to recover from the cancellations.
He says trains will not be in their normal spots so it is impossible to run the same services.
"We'll also have a driver roster that's changed due to what's happening tomorrow," he said.
© ABC 2013
20:14 EST As the sun set over Melbourne, a thick fog engulfed the city.