Groper Train: Tight Underwear (1997)

Original title: 痴漢電車 くいこむ生下着

Takeo, a university student, had a crush on Kaoru, an office lady who rides the same train to school. One day, Takeo found himself face to face with Kaoru on a crowded train. However, Kaoru was being molested and asked Takeo for help. But Takeo didn't have the courage to help her. After seeing Kaoru climax right in front of his eyes...