I remember Mythbusters tested a similar thing back in the day. They determined that you would need to be "unrealistically close" to the electrified rail (or whatever other source) to receive a shock. I don't know how close would be realistic..
The theory is that the urine stream would break into droplets at some distance.
I don't remember how far away we were, I think it was just regular "I'm gonna pee on that" distance. A preteen with a full bladder can indeed produce a steady enough steam to receive a brief jolt of "well, that's a bad idea" to the penis. Luckily the stream does break up, it would not be fun to be locked on till the bladder empties.
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