In tonights episode of "Sherlock" on PBS, there is a scene where Sherlock and Watson are in the Underground, walking through a tunnel. In the scene I could see the standard outside third rail, which I assume is used for power on the underground, but there also was a third rail between the track rails....was that some sort of weird thing the show dreamed up (I assume they didn't film in an actual tube), or does the underground use a third inside rail for some reason?
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Outside 3rd rail is power. Inside 4th rail is return.
Nick
Outside 3rd rail is power. Inside 4th rail is return.
Nick
I can't remember the exact voltages concerned, but I believe that on the 4 rail sections the voltage potential is split between the two rails. So if the total voltage is 600V DC, the outer rail could carry +300V DC, and the centre rail -300V DC. 
That sounds about right to me Nicole. If I remember correctly voltage it is not equally split between the two rails. The 3rd rail is +400V DC and the 4th rail is -200V or somthing like that.
I nearly got a engineering apprenticeship from sixth form at London Underground. Passed all the interviews and then the government at the time cut the funding for 18 year old apprentices.
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Nick
If you use a 4th rail, and both running rails have nothing to do with electrical current, you are less likely to have voltage loses.
One, the female doctor who was with SH said in the Tubes, the cars are called "cars" instead of "wagons" because of American influence. What American influence was there on the construction of the tubes?
I did a little digging, and there could be a number of things they were referring to. I suspect it had to do when they electrified the various lines in the years around the turn of the 20th century. For one thing, an American financier, Charles Yerkes, was involved with the lines, and it was his influence that they went to a 500VDC system using the multiple unit control system that Frank Sprague created (orginally the plan on the Metropolitan was to use overhead wire and three phase AC). I also ran across reference that the design of the subway cars in the Tubes was influenced by the designs being used on the IRT in NYC, there was a reference that the London guys studied the NYC system and liked the design of the cars on the NY system, so adopted a similar design (especially the roof) for their trains. That is what little I could find, in any event. Was was amazing is some of the lines according to the Wiki article I read were still running steam in the 1950's into the 1960's, though I assume that is the outside lines, not in the tubes....
The other interesting thing I read is the tubes are supposed to go 24 hours in 2015, joining NYC as one of the few with 24/7 service.
I hope those who watched this episode also noticed that the guy who was helping Sherlock with the video and details of the London Underground system was also a transit modeler with an HO subway layout in his flat....
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