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Is the Third Rail a Special Rail?
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Warwick
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Bigger and heavier. If you touch it it will bite you.
mallard
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More modern, "lightweight" third rails (such as those installed in parts of the Merseyside network or the East London Line) are, but most third rails are much older (as I understand it, they last substantially longer than running rails and some may be 100-odd years old) heavy steel bars.
furnessvale
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Even before composite rails, the third rail had a different metallurgical composition making it more conductive but useless for running on.
GRALISTAIR
Established Member
Yep absolutely
Mutant Lemming
Established Member
Sounds like a contest - 'Find the nation's oldest conductor rail still in use'. Think I'd dispute the 100 + years old claims as most of the Mersey and Euston-Watford were relaid and Southern electrification came later.
Dr_Paul
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Having travelled on the Southern, the LT Underground and other lines in and around London for decades, I've long noticed that the conductor rails are of a different profile to the running rails, and have sometimes wondered whether they are of a different type of metal on the basis of conductivity and the lack of weight being imposed on them.
I do find it a little hard to credit that, other than on rarely-used sidings, some lengths of conductor rail might be 100 years old: whilst they last longer than running rails, they nonetheless seem to get replaced on the lines up to Waterloo every decade or two, and it's quite easy to see that the rail heads get worn down over the years, not least when new lengths of conductor rail are put temporarily in the four-foot prior to being laid.
I do find it a little hard to credit that, other than on rarely-used sidings, some lengths of conductor rail might be 100 years old: whilst they last longer than running rails, they nonetheless seem to get replaced on the lines up to Waterloo every decade or two, and it's quite easy to see that the rail heads get worn down over the years, not least when new lengths of conductor rail are put temporarily in the four-foot prior to being laid.
apk55
Member
Modern conductor rails are probably larger cross section area than older rails so the resistance will be lower. After all running rail dimensions have increased and trains have become heavier and more powerful. I presume they pick a blend of steel with the best conductivity for the application. Composite rails with an aluminum core are an alternative but may be more expensive for the same conductivity depending on the relative price of aluminum and steel.
However the main limitation on electrification systems is the return voltage along the running rails. More than a few volts drop along the rails has safety implications as well as corrosion issues
However the main limitation on electrification systems is the return voltage along the running rails. More than a few volts drop along the rails has safety implications as well as corrosion issues
Steel with a lower electrical resistance than that used for running rails, was certainly in general use by the 1930s. This being noted in a pre-war textbook on electrical engineering that I have.
There is indeed a limit on the permissible voltage drop along the running rails, for electrical safety, prevention of interference with other services and to avoid corrosion. Remember that the running rails are accessible to the public at level crossings and the like. Whilst people usually wear shoes, there is no requirement to do so and the voltage must be low enough not to endanger persons with bare wet feet.
The obvious way to limit the voltage drop along the running rails is to reduce the resistance by use of heavier rail and by supplementing the rail with copper return conductors.
A more complex but much more effective strategy is to use what is known as a "negative booster" that in effect "sucks" the return current out of the running rails and forces it back to the substation along an insulated return conductor.
Historically this was a form of rotary converter consisting of a motor designed for a few volts at hundreds of amps and connected in series with the positive traction supply, this operated a dynamo on the same shaft that was connected in series with the negative return conductor.
When a train was in section and drawing current, the rotary converter would develop a slight negative voltage at the far end of the running rails.
The detailed arrangements were a bit more complex than that, but the intention was to limit the voltage between true earth and the rails, and to adjust this as the load altered. These days a similar result would be achieved without moving parts.
There is indeed a limit on the permissible voltage drop along the running rails, for electrical safety, prevention of interference with other services and to avoid corrosion. Remember that the running rails are accessible to the public at level crossings and the like. Whilst people usually wear shoes, there is no requirement to do so and the voltage must be low enough not to endanger persons with bare wet feet.
The obvious way to limit the voltage drop along the running rails is to reduce the resistance by use of heavier rail and by supplementing the rail with copper return conductors.
A more complex but much more effective strategy is to use what is known as a "negative booster" that in effect "sucks" the return current out of the running rails and forces it back to the substation along an insulated return conductor.
Historically this was a form of rotary converter consisting of a motor designed for a few volts at hundreds of amps and connected in series with the positive traction supply, this operated a dynamo on the same shaft that was connected in series with the negative return conductor.
When a train was in section and drawing current, the rotary converter would develop a slight negative voltage at the far end of the running rails.
The detailed arrangements were a bit more complex than that, but the intention was to limit the voltage between true earth and the rails, and to adjust this as the load altered. These days a similar result would be achieved without moving parts.
Joseph_Locke
Established Member
Normal steel conductor rail is 150 pounds per yard when new, wheres running rails are 113-ish (121 for 60kg rail).
The composite aluminium / stainless steel conductor rail was found to have a significant flaw (a stationary train pulling away at a station could develop arcing so badly that the aluminium body of rail melted out from under the stainless capping) so Merseyrail has had to re-instate all steel rail at many locations.
The LUL fourth rail is specified to be -50V specifically to deal with stray current issues.
The composite aluminium / stainless steel conductor rail was found to have a significant flaw (a stationary train pulling away at a station could develop arcing so badly that the aluminium body of rail melted out from under the stainless capping) so Merseyrail has had to re-instate all steel rail at many locations.
The LUL fourth rail is specified to be -50V specifically to deal with stray current issues.
I thought it was supposed to be -210V.
The actual voltage on LUL conductor rails varies a fair bit, but is usually considered to be a nominal 220 volts negative to earth on one rail, with a nominal 440 volts positive to earth on the other rail, giving a nominal 660 volts to drive the train.
In the VERY early days of underground electric traction, a simple third rail with return via the running rails was tried. This resulted in current passing through the iron tunnel sections and corrosion resulting.
A fourth insulated rail was then used, nominally at about earth voltage but insulated therefrom to avoid stray currents and corrosion.
Someone then suggested "as we have gone to the trouble and expense of providing an insulated return, perhaps we could energise this and thereby increase the total voltage available"
Thus was born the present rather odd system. The negative rail has to be at a lower voltage to earth than the positive rail because the insulators are smaller.
Negative voltages also tend to attract dirt and damp to a greater extent than do positive voltages, hence the desire to limit negative to earth voltages to relatively low voltages.
In the VERY early days of underground electric traction, a simple third rail with return via the running rails was tried. This resulted in current passing through the iron tunnel sections and corrosion resulting.
A fourth insulated rail was then used, nominally at about earth voltage but insulated therefrom to avoid stray currents and corrosion.
Someone then suggested "as we have gone to the trouble and expense of providing an insulated return, perhaps we could energise this and thereby increase the total voltage available"
Thus was born the present rather odd system. The negative rail has to be at a lower voltage to earth than the positive rail because the insulators are smaller.
Negative voltages also tend to attract dirt and damp to a greater extent than do positive voltages, hence the desire to limit negative to earth voltages to relatively low voltages.
Joseph_Locke
Established Member
Indeed, my apologies.
I thought that it was -210v/+420v to give a 630v potential?
apk55
Member
LUL works on a totally isolated traction system, with no traction current flowing in the running rails. The voltages may be referenced to ground via resistors and an alarm would be triggered if there was any significant current.
Trains should be regularly checked for isolation of the traction system from ground.
In some places where inter running is required with 3rd rail trains (such as Watford or Richmond) the return rail is then connected to the running rails. A long section gap is then required to separate the two systems.
Trains should be regularly checked for isolation of the traction system from ground.
In some places where inter running is required with 3rd rail trains (such as Watford or Richmond) the return rail is then connected to the running rails. A long section gap is then required to separate the two systems.
a few web-sites say that too
swt_passenger
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The SSR (Met/District/Circle/H&C) is also being progressively uprated to 750v (-250v, +500v), and allowed to rise to almost 900v with regeneration in action. So the pos rail can get to near 600v anyway.
There's a thread about it in the LU sub-forum here: https://www.railforums.co.uk/threads/lu-ssl-750v-upgrade.136973/
edwin_m
Veteran Member
The third rail network was similarly uprated so the third rail can reach about 900V with respect to earth.
KingDaveRa
Member
I seem to remember reading there's a few sections where the conductor rails are energised at 630v/0v, for some reason? Or did I make that up?
I also seem to recall a website where an engineer from LUL had written down all this stuff, but I can't find it now. There was tons of really nice details. I shall have to try and find it now. Edit: It may have been District Dave's site, but I'm not 100% certain.
I also seem to recall a website where an engineer from LUL had written down all this stuff, but I can't find it now. There was tons of really nice details. I shall have to try and find it now. Edit: It may have been District Dave's site, but I'm not 100% certain.
transmanche
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Defintely those bits that are shared with National Rail third rail services (Richmond & Wimbledon branches of the District line, Bakerloo line north of Queen's Park[*]) - unless they've been upgraded to 750v/0v.
[*]Well, technically from a point south of Queen's Park on the DC lines.
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KingDaveRa
Member
Aaah, I knew I'd read about it somewhere. Thanks. 
edwin_m
Veteran Member
Where third and fourth rail trains use the same track, the centre rail is bonded to the running rails so effectively at 0V. I believe the D stock wasn't permitted to run on 750V but the S stock is, so now the D stock has gone some of these shared sections are being uprated to 750V.
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