Sexual harassment and assault is a big problem on Japan's huge public transport system. One in 10 women aged 16 to 29 say they have been touched inappropriately on trains, according to the government’s first survey on the issue. But the actual number is likely to be much higher, as many women fear the consequences of reporting incidents to police. About 800 people are arrested for groping – known as "chikan" – in Tokyo every year. But many perpetrators get away with their crimes. In response, the police have launched an app to help them deal with the problem. FRANCE 24's Airi Futakuchi, Alexis Bregere, Mélodie Sforza and Justin McCurry report.