Sydney's trains are being fitted with sniffing technology to catch graffiti vandals in the act.
More than 30 offenders have already been caught under the scheme, which the state government is calling "mousetrap".
"This is innovative technology," said Transport Minister Andrew Constance.
"Mousetrap is on-board technology designed to scent the vapours of permanent markers, providing real-time information with the triggering of CCTV back to Sydney Trains staff and then, of course, also real-time information being provided to the Police Transport Command," Mr Constance said.
"Those who commit graffiti across the network can now be sprung immediately. With can in hand, marker in hand, doing the damage - you will get caught."
Sydney Trains chief executive Howard Collins said the system cost about $500,000. The state spent about $34 million removing graffiti last year.
However, Mr Collins remained tight-lipped about many of the details of the program, including how many trains were fitted with the sensors and where they were fitted.
"This is covert, you can't see them, there are many of them," he said.
"It's moveable, and therefore as far as we are concerned it is a technology that can be used in many locations."
The sensors do register other smells, and it can be difficult to differentiate between the vapour of graffiti or, say, someone transporting house paint. The sensors alert Sydney Trains or police staff monitoring CCTV footage, who are then able to see what has triggered the alert.
There were about 11,000 incidents of graffiti and vandalism on the city's rail system last year, and the problem is regularly cited in surveys as one of the major bugbears of Sydney commuters.
The sensors respond to the vapours released by spray cans and permanent markers. Once the sensors are triggered, they activate an alarm received either on mobile equipment held by police or by Sydney Trains staff.
A trial of the program started in the middle of last year, Mr Collins said, and would be extended to more locations.
