Chikan Play Densha no Naka de ○○ ya ×× na Koto

Toru has a girlfriend, Yuu. They keep good relationship and often have sex. But one day, when they are on the crowded train, their sexual desire suddenly explodes and they start having sex.... "It's great! More comfortable than normal sex...." They are gradually addicted to it.... Also, Yuzu, Toru's friend, and Sakura, Yuu's friend, get involved in it and a circle of train sex widens....