Germany's railway operator plans to deploy mini drones to catch vandals who deface its trains with graffiti, with the aerial vehicles shooting thermal images of its train depots at night.

Deutsche Bahn plans to soon start testing the vehicles which have four helicopter-style rotors and can shoot high-resolution pictures.

"We are going to use this technology in problem areas, where taggers are most active," a spokesman who asked not to be named told AFP.

A "pilot" remotely steers the vehicles at heights of up to 150 metres (500 feet) and speeds of up to 54 kilometres (33 miles) per hour. An operator evaluates the images, which the company plans to use as evidence.

Deutsche Bahn said it suffered property damage last year worth 7.6 million euros ($10 million) from people spray-painting its carriages.

The rail operator said it would only use the drones over its own depots, not in public areas, in line with German anti-surveillance laws.

Internet giant Google ran into problems in Germany when it filmed streets for its global Street View project, with thousands of residents complaining and Google being forced to blur many of the images.