Sex crimes on subways and trains have increased by 84 percent compared to three years ago, data showed Monday.

Out of a total of 3,568 railway crimes committed from 2012 to 2014, the number of sex crimes totaled 749 cases, or 21 percent.

The most frequent types of sex crimes on the railway were sexual harassment by touching (491 cases), followed by illicitly taking pictures without permission (235 cases).

In the case of illicit photos, perpetrators took photos with smartphones as well as hidden cameras in the shape of buttons, pens, keychains and adhesive cameras stuck on hats.

The railway police said it is monitoring activities through

657 security cameras across 40 major stations in the country, and created a task force to deal with sex crimes on trains.