As some of you may know, I recently returned from a two week trip to the United Kingdom. While some of my previous enthusiasm for London Transport has waned, I still must reinforce how easy it is to travel around the country when compared to the United States.
Across the next few days, I’ll post musings on various transit-related experiences. To begin with however, I want to thank our mates from Transport Blog, who we were lucky enough to meet up with over a cup of tea at a coffee shop near – where else – the London Transport Museum. It quite an enjoyable meeting, at which stories of horrible and great transit systems alike were exchanged, and at which we marveled at how politicians on both sides of the Atlantic could be so daft all of the time, even if we disagreed on the reasons why. Unfortunately, our goal to meet up with other UK bloggers went unmet, but we’ll certainly try for next trip.
One question struck both Randolph and me, however, that even the Transport Blog guys weren't sure how to answer. Instead of the three rails (two for the wheels and one electrified) on U.S. subway lines, the London Underground has four. Can someone please tell us why London needs an extra rail?
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amg | 1:40 AM |
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From the London Underground website:
UL use a four rail system to run trains. The two running rails are used for signalling and as a traction earth. The two conductor rails have a nominal pole to pole voltage of 630v DC. This voltage is then referenced to earth by the introduction of resistor banks (known as bleed resistors) for earth fault detection purposes only. Otherwise it is an earth free system.
The earth reference is Pos to earth +420v, Neg to earth -210v. Both rails are equally dangerous and can kill.
The advantage of this system is that you can continue to run trains under earth fault conditions. Should say a tin can get wedged under say the pos rail then an earth fault would be created. The positive to earth voltage would be 0v. The negative to earth voltage would be -630v. The train would still see the pole to pole voltage of 630v and operate normally.
On the mainline railways an earth fault on the positive rail would cause the circuit breaker to trip and the supply would be cut.
In addition to the above most track sections are fed from two sub-stations (one at each end). Should there be a supply problem at one sub-station then the other sub-station can still feed the section of track and trains will continue to run.
If one of the electric rails fails for any reason, all 630 volts will be transferred to the other rail, with no effect on the running of the train – and so no delays for passengers. On the main line a power failure to its single ‘traction’ rail would simply stop the train.
I looked this up yesterday. The encyclopedia's explanation was that it was due to fears of electolytic corrosion of tunnel linings and other underground pipes. Though that begs the question why it isn't done anywhere else.
The main answer to Patrick's question is that the Underground was electrified before people really understood the effects of electricity (also, being a bored-through-clay rather than cut-and-cover or bored-through-rock system, it's somewhat damper than most comparable networks).
Would it be possible to construct on conductor rail ramp from G.R.P?
> The main answer to Patrick's question is that
> the Underground was electrified before people
> really understood the effects of electricity
No, they were well enough understood at the time.
The reason for two conductor rails was to minimise electrolysis, as Patrick already noted above. Management of electrolytic corrosion effects remains a major issue for electric traction systems to this day.
Day for day I surf around in net to meet interesting people and international places. It’s great to see that it really works. Thanks J
