The union representing Auckland train drivers and crews has welcomed an inquiry into a graffiti attack by about a dozen youths at a passenger train stopped at a station.
The Transport Accident Investigation Commission has announced it is looking into the "unauthorised immobilisation" of the train.
The taggers struck at Baldwin Ave station in Mt Albert on Sunday afternoon, covering the sides of two carriages with paint while passengers were inside.
The Rail and Maritime Transport Union says the investigation is warranted because public safety should be paramount.
General secretary Wayne Butson says the incident calls into question Auckland Transport and train operator Transdev's plan to remove on-board staff and have only drivers from next year.
He says union members reported that the on-board manager prevented the incident becoming more serious by ensuring the doors remained locked and calming passengers.
Police are looking for the 10 to 15 youths responsible, most of whom had their faces disguised.
Sergeant James Cassin says the group spent four to five minutes in "a mindless, senseless action of vandalism" which was traumatic for passengers and staff.