Considering how packed some subway and train cars can get there, you can wind up being an unintentional groper.
A good friend of mine had a woman run from him screaming in one such situation. He'd stuck one of those folding umbrellas in his front pants pocket. Dunno what it felt like being up against that but I do know what it looked like... ;-)
Yes, I always carried something in one hand and had the other holding the rail/ring. Watching the strained faces of passengers when the trains lurched was always humorous; people falling all over one another. Not so funny in the hot, muggy summers.