I'm going to take a picture of a man on the train.
That's creepy. What are you going to do with the picture?
I'll show it around, what do you think?
You should not take pictures of someone without asking for their permission first. Most people don't like having their picture taken when they aren't expecting it, especially when a stranger does it. You will make the person feel very uncomfortable if you take their picture.
Fuck them, I'm not saying anything, it's just a photo.
Even if it is just a photo it's a violation of their privacy and you will be regretting doing it if one day they will end up pressing charges against you and you will pay a hefty fine. I do recommend you just delete that photo or at least try to get permission if it is okay for you to take photos of him.