Metro Trains lifted speed restrictrictions just after 7pm on Friday as temperatures across the city finally eased into the 20s, but restrictions remained in place on the V/Line network until 10pm.

Blanket speed restrictions were implemented on the Metro network once the mercury hit 42 degrees on Friday afternoon.

Speed restrictions for V/Line services kicked in once the mercury hit 39 degrees on Friday, knocking top speeds of 130km/h down to 90 km/h.

Bus replacements and a number of cancellations were in force on the Wendouree, Bendigo/Swan Hill, Geelong/Warrnambool, Seymour, Shepparton, Traralgon and Bairnsdale lines during Friday.

It followed distressing conditions for passengers on Thursday who were trapped on at least one slow-moving train with little to no air-conditioning after heat-induced signal faults on the Geelong V/Line track.

Some passengers broke open a carriage door near Lara and climbed down onto the tracks out of frustration.

"In these conditions track temperature is significantly higher, and steel railway tracks expand in the heat," V/Line said in a statement.

Yarra Trams also warned services could be impacted or cancelled during extreme heat, with a number of lines experiencing delays and bus replacements.

In unrelated maintenance works, the Sunbury, Craigieburn, Upfield, Werribee and Williamstown services will not travel via the City Loop this weekend, starting 9pm Friday.

Passengers will need to change services at Southern Cross or Flinders Street station to access the City Loop via other lines.