Sydney Trains staff are reviewing hours of security video hoping to identify graffiti vandals who staged a dangerous stunt on the Harbour Bridge.

7News has obtained exclusive pictures of the pair spraying their tags on the outside of a moving train.

Some friends were passing in their car when they noticed the vandals.

They videoed the men going inside the train then opening the side door and tagging it.

Sydney Trains' new boss Howard Collins says security video will be handed to police and he's confident of arrests.

"I daresay these people have relatives or friends or parents, who want to tell someone that they've just been killed by doing something stupid," he told 7News.

The penalty for graffiti offenders is a fine of up to $2,200 or 12 months' jail.

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