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.
A container smoulders after firefighters put out the blaze. Photo / Supplied
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.
The blaze broke out at the Onehunga depot shortly after 10:30 am. Photo / Supplied
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.