The scene: Around midnight in a train leaving a major city towards the smaller towns in its outskirts. A couple sits down in the last group of seats of that wagon, my (non-kinky but indifferent) friend and I sit in the group next to the opposite window (~3m away), no one else in sightline of anything below her shoulders. She hides her intimite area with a bag, but he is clearly fingering her (I'm absolutely certain because of her trying to hide her heaving? and the fact that he wiped his fingers off her with her bra afterwards).

How would you have reacted in this situation?

I contemplated a few things:

1. Acknowledge that you see but that it's ok. (A nod in their direction for example.) If they get their kick out of the "they might know" or "we're doing something forbidden" then this would spoil the scene. If they're driven by more explicit exhibitionism they might get a better scene by being allowed to play more overtly. (They both wore masks due to mandate, btw.)

2. Don't tell them that you know. Regardless of if they saw you looking, they could think that they violated your consent and / or they get their kick out of it.

3. Tell them afterwards / when they leave the train that you knew exactly what happened (and that you're ok with it). This wouldn't spoil the scene, just recontextualize it afterwards and they would have the affirmative consent of us afterwards.

4. Tell them it's a no no to do anything in public even if you do this as hidden as possible, in silence, fully clothed and close to midnight. (As to how hidden it was, if you didn’t look to closely he might as well just had a hand or her thigh, like I said everything till his wrist was covered by a bag she placed purposfully.)

In the end I missed my opportunity to do 1 or preferably 3, so I basically chose option 2, but I would like to know what you lot would've done.

(I'm also 99% certain there was also a dynamic going on by the way they acted during and especially afterwards, but that's beside the point.)