Metro trains has revealed the latest high-tech weapon it is using to catch graffiti gangs in the act.

Vandalism costs the network a whopping $10 million a year, and 7News has been given exclusive access to the security camera bunker where the vandals’ every movement is being tracked.

While vandals are filming it themselves, they are also being watched from Metro’s new high-security nerve centre which monitors almost 5000 cameras.

“It’s constant feedback, its instant and its live,” Ron Bria from Metro Trains said.

Metro has invested in a new technology which senses when paint is being sprayed at graffiti hotspots.

“There’s sensors within areas that actually sniff for solvents, so paint solvents, and if that’s detected, it sends an alarm and an image to the control room,” Mr Bria said.

They are also using camera software which, over time, learns patterns of normal movements, then sends an alert when something unexpected happens.

“We don’t have to watch the 4800 cameras to know something’s happening, because we’ve now got the smarts to tell us when things are not quite right,” Mr Bria said.

Stolen Metro vests can be bought online and are worn to elude authorities, but police are monitoring social media accounts to predict when they will strike next.

“We’ll get the intel, we’ll believe we’ll know when and where something’s going to occur,” Inspector Darren Cooper told 7News.

“We put ourselves in a position where we can make the arrest.”

And police actually want gangs to keep posting their images online.

“The more visible they make themselves, the easier it is to detect them,” Insp Cooper said.

Cameras across the network monitor trains car parks, depots, the city loop and every one of Metro’s 223 stations - and they are always watching.

“We’ve definitely winning the battle, we’ve taken some really big gangs off, some interstate gangs off,” Mr Bria said.

Police have warned they will continue to find and prosecute offenders.