If you arrange it properly it can work out. As stated you can all agree on a time, which train and cart (such as the last one for example), and a way ensure its the correct person (such as wearing one distinctive article of clothing). It takes more effort than regular dogging or public play, but not impossible to execute.
Just to let you know, all the subway cars have cameras, so you would be on film, lol. But your fantasy has possibilities, done in the right context. It would have to be rush hour, the largest crowds are at those times, and you would more likely have a few voyeurs watching. And this day of multi media, it would be posted in about five seconds. Well good luck. Happy Hunting lol.