So I'm catching the last tube home on Saturday night, and this dude staggers past me, barely able to walk. He careers to the end of the carriage and opens the first of the connecting doors. He then whips out his ding-dong and proceeds to urinate into the gap between the two doors. I suppose we should give him points for politeness- he could have just wet himself and had his urine play chase-the-fellow-commuters up and down the passenger car.
But, that aside, I really was prepping myself for the guy to go flying backwards, a charred sausage in his hands. Could someone explain the physics of this to me: if you piss on the live rail under the train, you're in a hell of a lot of trouble, is this not so?
This is a 
I think in the tube station the live rail is the one on the far side, not the one in the middle.
The gap under the one in the middle, is so that if you get knocked in when a train is approaching you can lie there and not get run over. Or so I was told.
Also pissing on a live wire won't necessarily fry you. It depends on the voltage as to whether it can travel up your urine, the amplitude as to whether it will fry you, and the contents of your urine as to how easily it will conduct.
Well surely those factors remain a constant? I'm texting AQA about this.
Well the line voltage is 600V, and the tube cars draw about 180 amps from them.
Whilst this is more than plenty enough to kill you, it's very unlikely that it would be enough to flow up your urine. Firstly urine isn't a constant stream as any stream of liquid will tend to widen towards the stream end, so the voltage would need to try break down air as well which it isn't strong enough to do.
The conductivity of urine is high enough (10mS/cm) for it to conduct the current. However one would assume the minute you felt the shock it would shut off the stream. If you've had a lot of salt or acid in your system you're more likely to get shocked. More sugar, less likely. Water itself is not a great conductor.
Well, I've certainly been zapped after whizzing on an electric fence. (Yes, on purpose. It's a farm thing).
But did you get hit with the full shock, same as if you'd touched it?
Moreso when it comes through your wing-wang.