Fire crews were also battling a freight wagon fire in Onehunga.
Stevenson said eight fire crews have been sent to a fire in a rail yard in South Auckland.
Four containers on one carriage of a train were on fire, but the fire was contained and not expected to spread. The train was not a passenger train, Stevenson said.
The alarm was raised at 10.45am.
Auckland Transport said it isn't a passenger train so there are no disruptions to passenger services.
Spokesman Mark Hannan said it is a KiwiRail train at Westfield yard.
Anthony White is with a dozen people in an abandoned carpark just off Mount Smart Road, overlooking the railway lines.
He said black smoke was billowing all across Onehunga and you could see it from miles away.
White said it was easy to find because it was a massive black plume.
Traffic is building in the Onehunga area which is busy with a Santa parade taking place.