Like other cities across the globe, Sydney, ?Australia, has spent huge amounts of money battling graffiti writers who deface buildings, public transport vehicles and even monuments.

The city's fleet of commuter trains bear the blight of graffiti scribbled with markers or splattered with spray paint. While surveillance cameras help, there are too many cars to monitor at all times, so officials here are hoping a new surveillance system based on an "electronic nose," introduced late last year, will prove more efficient.

“We've had 50 people being charged with offenses," said Howard Collins, CEO of Sydney Trains CEO.? "And as we roll it out now to other trains, it's proving even more successful.”

Called Mousetrap, the device — developed by the Australian company Technique Risk — can detect fumes released by markers and spray paints. It quickly compares them with known types of paints and triggers an alarm connected to the nearest surveillance camera, says company founder Mark Byers.

“The strength of the system is the fact security personnel or police can monitor the system in the field with a mobile phone," he said. "The information that they receive is the train or carriage number, the location the event is occurring and also where that train is going to."

Vandals cannot disable the system because the sensor is hidden in the train’s wall.

Undercover police officers, who patrol the trains and platforms, can quickly react and apprehend the vandal in the act of defacing the train.

Collins noted the system has some additional advantages.

“Now we're building a profile of where and when these incidents occur, so the police already have the intelligence and the information to say, 'this is where this is likely to occur and this is the time of day,'” he said.

Collins said that thanks to the Mousetrap, most of Sydney’s trains are now graffiti-free. Installing the system cost about half-a-million dollars, a small sum compared to the $34 million Sydney Trains spent last year to remove graffiti.