• London Underground 4th Rail

  • Discussion about railroad topics everywhere outside of North America.
Discussion about railroad topics everywhere outside of North America.

Moderators: Komachi, David Benton

  by phillyrube
Started watching some video on youtube, about the Underground. Noticed a 4th rail, used as a ground, for the power. Any ideas why, since we widely use the running rails as the ground?
  by David Benton
I have been asked this before , and found the answer, but can't remember what it was.
Found this answer , but I'm not sure it is correct.
https://www.quora.com/Why-does-the-Lond ... four-rails" onclick="window.open(this.href);return false;
  by Benny
Also Milan metro line 1 has four rails.
The official reason was to avoid that eddy currents damage the main rails but unofficially the choice was made to ease an eventual future conversion to rubber tyres use (in the 60s there were strong pressures by some industries to put on use a system called "strada guidata" (driven road)

Ciao :wink:
  by phillyrube
Found anothr youtube that explained it in some depth. I'm guessing that since the "neutral" is insulated, there would be potential on that as well, similar to USN ships.
  by David Benton
That's what I am thinking too. If the middle rail is -260 volts, and the running rails are earthed ,then the running rails are 260 volts positive to the centre rail. Certainly enough to prove fatal.I always thought the refuge hole in the centre of the tracks was a safe place to fall, but not with 260 volts D.C in the middle.