Answer:
Used left or right I get to travel over cobblestone or gravel. Used up I vie for sweet success used down I cause men great duress.
As I went over London Bridge I met my sister Jenny; I broke her neck and drank her blood and left her standing empty.
I am a protector. I sit on a bridge. One person can see right through me, while others wonder what I hide. What am I?
I save lives on the ground and in the air
Sing me to a baby about to go to sleep.
The answer I give is yes, but what I mean is no. What was the question?