"chikan" is the closest we have I guess. It's basically public sexual harrassment, and it often happens on crowded trains. But yeah, a specific "train/subway" is much better and it doesn't even imply harassment or rape
anyway I've already mentioned it a few times but I think it would make sense to create a "settings" type for this kind of tag. Other examples would be "school", "beach", "home", "fantasy", "workplace", etc.