A mobile-payment system meant to help German commuters buy transit fare has also invited fraud from people prefer to ride for free.

The new system allows riders to buy tickets with a mobile device, The Wall Street Journal reported Friday. German transit systems work on the honor system, the article said, though the transit providers do random inspections to make sure people are bringing valid tickets when they ride.

Fare dodgers are using technology in two ways to avoid paying for every ride. They are using social media to warn one another about where the inspectors are. They are also using a German smartphone ticket-buying app to buy tickets only when they are about to be caught by an inspector, the article said.

Lars Wagner, a spokesman for the German transit agency VDV, told the Journal that although there has not been a spike in fraudulent riders, "this is a trend that is not so positive."