Passengers were moved to a train's rear carriage to avoid fumes when the train was tagged this morning.
Auckland Transport spokesman James Ireland said staff were reviewing CCTV footage after the train was tagged at Henderson about 5.50am.
AT would provide the footage to police if need be.
Western line commuters were told over loudspeakers the citybound service would not be stopping at Newmarket station because of the tagging.
Passengers were told to only board the train via the rear carriage because of fumes.
Ireland said the train had since been taken to the depot in Wiri to be cleaned.
"Tagging trains is unacceptable, and we take it very seriously," he said.
Several months ago passengers on a train bound for Britomart were traumatised when a masked gang hijacked the vehicle and spray-painted a "graffiti bomb" on its exterior.
The group hit the train's safety button, stopping the train, and the carriage quickly filled with fumes from the spray cans.
The train driver locked the doors shut to keep passengers safe until police arrived.
"That's the safest place for them to be - the last thing we want them to do is go out there," the spokesman said.