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?
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;