A passenger train carrying 89 passengers travelled 59 metres past a stop signal while approaching Plimmerton Station before emergency braking brought it to a halt.
The Transport Accident Investigation Commission has opened an investigation into the 1 September incident involving a Transdev Wellington passenger service.
Southbound Train 6219 was travelling towards Plimmerton Station on the North Island Main Trunk line when it passed signal 803 while the signal was displaying stop.
Emergency braking was applied and the train stopped about 59 metres beyond the signal.
There were 89 passengers and four crew members on board.
No passengers or crew were injured, and no damage to the train or railway infrastructure was reported.
TAIC said there were no conflicting rail movements at the time.
Passing a railway signal displaying stop without authority is known in the rail industry as a signal passed at danger, or SPAD.
TAIC investigates transport accidents and incidents to establish what happened, determine contributing factors and identify lessons that could improve transport safety.
The Plimmerton incident has been listed as Rail Inquiry RO-2026-107.
The investigation is continuing.