I think most manhwa only readers won't know this. So I will go ahead and say, Train Molestation and Stuck-in-a-Hole to be used by Strangers Sexually are both a kink in Japanese BL/Hentai/Porn(never seen one for GL). Like there are even dedicated tags.
This may be the first time I have ever seen such kinks in a serialized work and not a dj or one-shot, and both tags at that. Maybe author-nim have come across them a few times and inspired by them.
I'm not personally that comfortable with both and I avoid them but if you're into BL long enough you end up seeing what you don't wanna see one day.
Once I felt like hey all this black suspect figure and gradually growing list of connections are giving strong Detective Conan vibes, I can't help but read the deductions with the dramatic effects as in the anime
shinjitsu wa itsumo hitotsu