I see people having sex on the train and platforms all the time. I'm an overnight stock person at Duane Reade. Usually when I see them they see me, get surprised and stop. It’s happened so much that I just attribute it to the risk of having a night position.
In 2009, these two teenagers (or adults) were having full-blown sex on a W train. I was looking at them through the glass at the end of the car. She had on a dress, with her panties around her right ankle, and he was naked from the waist down to his shoes. They kept going at it from Broadway where I got on, to 49th Street where others started getting on the train. I was certain they were going to get caught at 59th Streeet, but surprisingly no one got on there (though someone from the platform did take a photo of the two lovebirds).