The Transport Accident Investigation Commission has found a maintenance issue led to a Wellington train derailing with 315 people on board.
The train's rear carriage jumped off the track just near the Kaiwharawhara depot in May 2013, injuring four people.
The commission said part of the train's braking system fell onto the track and jammed the underslung machinery.
It said maintenance staff had failed to fit the brakes properly more than 10 weeks before the accident, because of a shift change.
The commission said it made an urgent recommendation to KiwiRail to address the way maintenance was conducted.
KiwiRail said it had changed its procedures following the investigation.
It said changes included improving the communication between staff, with sign-off sheets and secondary checks now required.