Train services in Wellington are expected to be mostly back to normal today after a major fault yesterday.
Hutt Valley and Johnsonville trains were cancelled because high temperatures caused a point fault, and affected wires on the line.
Trains on the Wairarapa and Kāpiti lines were delayed as a result - and about 8000 commuters had to find another way home.
Chief operations officer Henare Clarke said KiwiRail was extremely apologetic about the delays, but it was a safety call.
"What happens with these extreme temperatures is the lines themselves start to expand, and they start to sag and so what our engineers do, they need to make an assessment of whether the trains can run safely underneath those."
He said engineers would continue to monitor the lines, although the heat was expected to be less.
"We know that the hottest part of the day is between 3 and 4pm, and we will be increasing our monitoring to see what's happening with those lines."
He said the lines had not been affected like this before, but the lines were already undergoing an upgrade - set for completion by June 2021 - which would prevent the problem in future.
"This has not happened previously ... over decades we've not seen these sorts of extreme temperatures.
"The lines are going through an upgrade, we've received funding from the government and what that will do is take away the issue around heat susceptibility and also future-proof it so that if there is an increase in commuter traffic the upgraded lines will be able to handle it.
"What actually happens is they have what they call auto tensioners which means the lines are continually tensioned, and the upgrade will provide that.
Metlink this morning said most trains would be back in action for peak services.
Metlink advised commuters to check online for the latest information as the day warms up in the afternoon.