A faulty train or power surges may be why electric trains on one of Auckland's busiest rail lines were grounded for 10 days, Mayor Len Brown says.
Auckland Transport was investigating, he said.
The "intermittent power fault" occurred twice in a day and led to electric trains being temporarily replaced on the Onehunga Line from July 23.
Chief operations officer Greg Edmonds said there were no safety issues associated with the incidents.
"We need to get to the root of the issue but just as importantly we must ensure services run reliably for our customers," he said in a statement at the time.
"So as not to inconvenience customers we have decided to run diesel units."
Timetables were unchanged.
Brown told Fairfax Media this afternoon there was a "possibility of one of the trains being faulty" and "possibility of difficulties around power surges".
"So I want to be sure about Auckland Transport's final report because this is critical.
"If it's about power delivery then that is systemic and we've got a problem."
If the issue was about one train "that wasn't quite right then we need to really be laying [it] at the feet [of those responsible] and be really, really clear around their delivery.
"We haven't had a finalised position on that," he said.
"We need to make sure that analysis is absolutely spot-on. I expect that in days, not weeks."
Knowing the exact issue was "critical" due to how busy the Onehunga line was, he said.
"I'll expect people to be doing yeoman's hours to make sure everything is all right."
The fault caused delays but the electric trains were now back, Brown said.
Edmonds said minor issues such as the fault were to be expected during the commissioning phase of the new train fleet.
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