KiwiRail says it's working hard to upgrade train tracks so they won't be as affected by heat restrictions.
It comes after some Auckland train services had to be cancelled Monday as speed restrictions were put on when the tracks became too hot.
KiwiRail executive David Gordon told Morning Report 4km of the approximate 200km track network were affected.
"Those heat restrictions were all in an area where we haven't yet had the opportunity to do the closure to do the works."
He said work was done at 80 sites over the Christmas holidays to upgrade the tracks and make them more heat resistant.
"It would have been better to have them all done," Gordon said.
"If you're a commuter you don't care whether it's heat restrictions or something else, you don't care whether the trains got a problem, you just want your timetable on time.
"Quite frankly at the moment they haven't got enough reliability. We recognise that and we're doing our best to fix it," he said.
"We can't control the temperature of the sun, what we can control is the quality of the track as much as possible and clearly in this one we've let the side down."
Gordon said such heat restrictions were in place all over the world when track temperature rose above 40degC.