Chikan Densha Kyonyuu Otori Sousakan ~Nerawareta Ureta Nikutai~

The protagonist Mizuse Asami is a member of a secret government organisation. She deliberately baits serial gropers dressed in provocative clothing and then arrests them after obtaining incontrovertible evidence. Her organisation has a huge impact in enforcing the contemporary anti-groping laws.
One day, Asami boards a train as part of her undercover investigations. She's once again the target of molesters but something is different from the usual treatment. The overly brazen molestation escalates and all of a sudden other passengers participate in her violation.

All of the perpetrators turn out to be gropers who had previously been arrested by Asami for their crimes. They are now working together to violate Asami to enact their revenge against her.

Violating Asami on the train isn't the end of their vengeance, rather it's just the prelude. The gropers force Asami off the train and confine her in a room...