It seems to happen quite frequently these days, and 99% of the time it's men. It's disgusting, I know sometimes it's not deliberate but leaving a stench like that for other people is vile, and you can't always open windows on trains, like the Avanti one I'm currently on.
If it's not that, it's people chewing their nails and then wiping it on whatever they find, picking their nose, generally disgusting habits to do on a packed train. Again the vast majority are men.