Prototypical 4 Rail?

To me, it looks like regular standard gauge 2-Rail, with an outside electrified "3rd-Rail", plus another inside electrified "3rd- Rail".

I for one, would like to know the logic behind this, unless the British isolate the return from earth ground.

Rusty

My memory is slipping.  IIRC, this is either the Bakerloo line or Jubilee line.  If so, I rode it every day for years.

ChipR

Great. Then are you able to explain the unusual powered "3rd-Rail" configuration?

I read somewhere (possibly wikipedia) that it was to prevent stray return currents from inducing electrolytic damage in surrounding water and gas pipes, as neither those nor the subway tunnels themselves were built with electrical bonding between pipe/conduit/tunnel ring segments.

As for the +420VDC and -210VDC system, I haven't figured out how they induce the negative potential in the center rail.

---PCJ

I've seen the picture before long ago...interesting huh?

Hot Water,

Sorry, I can't.  The underground was just a way to get to my cli ent's office.  Now, the preserved steam railways and BR were a different story.

ChipR

What?  Just four rails?  Have you seen that book on Cincinnati traction, showing the

(spaghetti) junction of several different gauges of electric lines?

I had done some research on this, when I saw the 4 rail track on "Sherlock" on PBS. Basically the two extra rails are the power rails, one is the positive, the other the negative return. From what I read (which was a website, so no way to know if it was true), the idea was that a separate return rail worked better than using the track, plus the writer also claimed it also allowed signal systems based on the rails to work better (again his comment). 

It was interesting, originally they were going to use overhead AC, but Charles Yerkes from the US got involved with the underground, and he had experience with Sprague's MU power setup, and made the decision to go with DC. 

I believe there are some prototype railroads that use an inside third rail for power, and the closest thing I can think of in the US was the Manhattan and Bronx trolley lines, that used a third rail in a trough between the tracks to power them, rather than overhead wire. 

Those "extra" rails are insulated positive and negative terminals. The regular rails are not used as a ground. In New York City, the center slot was not used as a "third rail". There were in fact two rails within each slot, and the plough, with a sliding button on each side, would use one rail as a positive and the other as a negative. There was no "third rail". This was to prevent electrolysis.

Here is some more, but this time from Liverpool.

The photograph shows the gaps in the centre and outside conductor rails for the pointwork quite well. It's been around for quite a long time now, as this photograph is just over 100 years old. 

Yes, I have not figured out why the Tubes have differnt voltages on each of the electified rails.  Different polarity, yes, but vottage.

Voltage leakage can be an issue.  Houston METRORail has had it (750 overhead) and there have been issues on the NYC subway network.

The voltage difference is intended to help reduce the adverse effects of leakage into utility pipes and cables. This link explains the reasoning in more detail.