It was ``really difficult'' to say what may have caused the derailment, he said, adding nothing had been ruled in or out at this early stage.
However, he assured that driver fatigue was not an issue, saying the company had ``very strict fatigue management guidelines in place''.
``All I can say is that we're obviously going to work very closely with Transport Accident Investigation Commission, NZ Transport Agency, KiwiRail, and we'll conduct a thorough investigation of what went on,'' the Transdev managing director said, vowing the company would ``get to the bottom of it''.
The train had derailed as it was crossing lines at the busy junction, with the locomotive part coming off the tracks and ending up on its side, while the following two carriages remained upright.
``We've had a bus contingency in place, it's been a little more difficult with the Bruce Springsteen concert again at 7.30pm tonight,'' he said.
``At this stage we're hoping to get access back to the track at 4pm, albeit with a limited speed restriction of 25km/h over that section of track, but that will allow us to run services up from the south. Services from the west and the east will be unaffected, and we'll still be delivering folks at Penrose for the concert.''
He advised concert-goers to allow a little extra time for their journey as trains would be travelling more slowly than usual. A bus contingency will be put in place from Otahuhu to Mt Smart if the ``worst-case scenario'' happens and services have not resumed in time for the gig, he said.
However, passengers may see a disconcerting sight as they pass the train junction, as the derailed locomotive will still be lying at the side of the track. It may be Tuesday before it's possible to remove the locomotive from the area, Mr Scott said, as the company would have to cut power to the overhead cables to allow a crane to lift the vehicle out.
Early reports of a fire or explosion following the derailment were inaccurate, Mr Scott said, however there might have been a small diesel spill at the time.