KiwiRail put the safety of passengers and crew at risk when a tripped circuit breaker brought Auckland's rail network to a standstill, a Transport Accident Investigation Commission investigation has found.
The report investigated the tripped circuit breaker and subsequent shutdown, which stranded up to 2000 passengers in stationary trains on 26 April 2012.
The power shutdown was caused by a poorly designed electrical circuit feeding electricity to the train control room in Wellington.
Weaknesses in the circuit meant the fault was able to affect the entire Auckland train network.
The report said the electrical designers lacked expertise, and that the emergency power supply was not sufficiently managed or maintained.
As a consequence of the power failure, all light signals automatically switched to red, instructing trains to stop.
The controllers located in Wellington were unable to communicate with the stranded trains to inform the crew of the problem.
Although the situation was well-controlled, the commission said there were consequential risks for passengers stranded in immobile trains.
KiwiRail had resolved the issues identified in the report, the commission said.
KiwiRail acting general manager of infrastructure and assets Dave Gordon said it was a basic failure but responsibility for the incident should be shared with the electrical contractor.
"We did not provide [an installation manual] to the designer of the [electrical systems] and the other way to look at it as well, is to say, the electrical designer did not make inquiries to see what the UPS manual said when he was designing something that had UPS in it," Mr Gordon said.
Passengers could be assured by the company's generally reliable service and since the shutdown, KiwiRail had installed new electrical systems which have run without a hitch, he said.
Auckland Transport, which oversees the city's rail network, declined to comment but said it agreed with the report's recommendations.