Lay off the sauce, Hammie, you're looking unmanly.
But, yeah, the main reason for LU's fourth-rail system was the risk of electrolytic corrosion of metal pipes and other things inside or near the tunnels. Wouldn't do to have a gas or water mains spring a leak, now would it?
>>131166 -- For some highly technical reasons I'm not awake enough to remember or understand, the conductor rails must be at different heights above the surface. Short-circuit avoidance, I think. So, a differential supply will balance out the leakage currents and reduce corrosion risk.
A semi-floating two-rail supply means a 'window' detector is needed to check if the rails are at the correct ground potential and, if they aren't, to shut off the supply and ground the rails. Whether they use resistors or rely on the leakage currents to set the potentials, that I do not know.
PS: Grounding either supply rail to a running rail is an option available to LU drivers and staff to shut off traction current in an emergency.