Ahh-ha! I image-googled that phrase and came up with a picture of what I was considering placing a cache on. I've found caches on these before.
You do realize that the voltage inside of those transformers is usually 4160 volts before it gets stepped down? A shock at the voltage will kill you, I saw it first hand when someone jackhammered into a 4160 line that fed a green transformer.
It depends if you get an empty one you will be fine but when the electric company gets their maps that show wires nearby they will open it up. Have fun with the cache!