Man'in Chijo Densha ~ Hosoi Yubi de Shigokanaide

The main character is an ordinary, slightly short boy. One day, running late for school, he rushes into a female-only train car. Two women assault him, forcing him to ejaculate. The next day, the same women corner him again, this time allowing penetration. Crying, the boy confides in his beloved sister, who vows to track down and take revenge on the abusers.