you have to have sex 16 separate times.
Viewing post in Loop Train: Having Some Fun with Her and my Hypno-App! comments
okay im not sure the exact trigger but i finally got it.
i had her facing the train door and gave her headpats and stroked her cheek until orgasm.
I did have all achievements by this point, i think you need to have had sex with her before. on a new day give her headpats/caress her cheek before doing anything else