Train sex is a phrase of recent origin. It arose in places with overcrowded trains, especially Japan and India, where "accidental" bumping of hips, bottom and genitals developed into obviously deliberate molestation, and finally penetration.
The practice has spread to other public transport and other crowded areas such as concerts, and has spread to other countries as well. Young schoolgirls in uniforms are especially targeted.
One factor for its development included the tendency of many Japanese women to be subservient or submissive and not to complain or protest, to avoid embarrassment. Other factors were the loneliness of both the molester and his victim, and the gentle persistence of the molestor until his victim was aroused.
In Japan, a male who molests and initiates sex on trains is called a "chikan" and a female who does so is called a "chijo".
I'm so sad. I rode back and forth on the Tokyo trains all day, hoping for train sex, but I didn't get molested once!
I'm sorry officer, I was inspired by the fact that there are so many videos of train sex on the internet, and I thought I'd try it too.
Teacher, I was late for school, because someone started molesting me on the train, and I didn't want to get off at my stop, in case he wanted train sex.