The leashes can be dangerous. When my father was in the subway in DC, some cow eyed idiot housefrau came in with her spawn on one of those. She forgot about him and the door closed on the leash with her boy outside and her on the inside. She didn't notice until my doddering father was trying to jam his rickety arms in the subway door trying to pry the door open. To his credit, a young, alert(and much stronger) african american male helped my father pry the door open and throw the leash outside just as the Subway was pulling away.
The moral of the story? Don't get one, pay attention to your little boogers yourself.