On most sections the side rail is +420 V and the central rail is -210 V.

The sections with mixed service with National Rail have +630 V on the side and 0 V on the central rail (the running rails are always 0 V obviously)

So the return rail can indeed be negative or neutral, depending on where on the network it is.

Historically grown systems can be retarded sometimes I guess.

>stupid 4th rail
>"nut real"
>"negitive"

Yeah, it might seem stupid now, to you, but back in fucking 1906 and shit electrolytic corrosion of the steel tunnel segments was a legitimate and serious concern. Having the return current passing through the running rails and potentially onto the tunnel wall was therefore no good.

Additionally, the voltage is split +ve and -ve across the two conductor rails to reduce leakage across any dirty insulators or debris that falls into the tunnel (remember, once it goes in, it stays there until literally someone comes and removes it), so potentially there could be significant current leakage through parts that you really don't want to corrode (y'know, pieces of infrastructure that were LUDICROUSLY expensive in the 19th century when labor was cheap, and haven't exactly gotten cheaper since then if you need to rebuild them or do major overhauls).

But sure, Ham, come over here and teach us how to trains.

>>131160

This being a troll thread aside, does anyone know the reason why there is exactly 1:2 absolute potential difference between the running rails?

If I had to guess, it would be the resistors that are used to anchor the potetials to earth, so three resistors of same capacity would suffice, but then, why not use 1:1 split service?

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.

>>131163 If the tunnels are made of steel or iron awesome. There is a larger surface areia to go through and less likely there would be a gap in conductivity that would cause the current to wander and cause corrosion.