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Does LU need the Fourth Rail?

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edwin_m

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Hmmm ... so one wonders why on the "zero negative" sections it was even put on insulators in the first place.

When were the BR trains on the DC line (and the substations) converted from 440+/210- 4th rail to full 650+/earth?
If the BR/LMS trains were fourth rail, wouldn't that indicate that the route was electrified to LU fourth-rail standard with the third and fourth rails sitting on insulators and isoiated from the running rails?
 
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S&CLER

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I think that's an answer to why the 4th rail was retained at all, which is fair enough, but was wondering why the trouble was taken to drop it down to the sleepers and remove the insulators.


Shame I never saw this. We moved from Somerset to the West Kirby line just a few years after the old sets were gone (some were apparently not quite as old as they looked - a couple of the Mersey cars were actually built as WW2-destroyed replacements after 1945, and were newer than the LMS units).
I never saw the old Mersey cars either, though we lived in Merseyside (Maghull) and visited the Wirral from time to time, but usually by ferry. I was 8 when they were withdrawn and have no memory of them at all. Towards the end they may have been used mainly at peak hours. Car no. 1 was preserved, but destroyed in a fire at the paint shop in Derby, a grievous loss to preservation, since the Mersey was the pioneer British steam line to be electrified in 1903.

Some 4-rail Hammersmith and City stock, about to be scrapped by LT, was reconditioned for use on the Mersey during the war, in case of bomb damage, but never used (details in an appendix to Maund's Wirral Railway).
 
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Taunton

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We seem to be getting a bit mixed up here. As I understand it the DC Lines, Watford and the North London, were LT-style +/- 4-rail, right through to Euston/Broad Street, which aided how the Bakerloo was extended out over it, and indeed the District running out to Richmond. This applied to the LNWR stock, and the replacing BR 1957 stock. Some time (1970?) the BR sections were converted to normal 3rd rail, with an earthed 4th rail where LT trains still shared the tracks.

The whole thing of stray earth currents does seem to be a "dark art" - at least to some. Although the DC lines had been there long term, when the 25Kv was extended in the 1980s over the North London 3rd rail, a whole series of such issues arose; in particular there were problems with the Victoria Line signalling, including blown LT fuses, where it passed well underneath at Highbury. One (well informed) electrical engineer referred the (doubtless also well informed) project engineers back to a classic electrical textbook, by Dover, published about 1914, which apparently describes just the issues they had run into.

The most surprising account I saw was from Stanford University, near San Francisco, in about 1970, where the biology professors had devised an experiment for measuring electrical resistance in plants, with very small microcurrents. Some interference was spoiling the experiment, they initially suspected the high-power Stanford Linear Accelerator atomic research machine, run by their colleagues, but found it was not so. Their only clue was it was likely man made, as it only happened on Mondays to Fridays. Eventually they traced it to the initial trial runs of the BART rapid transit, 1,000v DC with running rail earth return - the nearest point being about 5 miles away!
 

AM9

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We seem to be getting a bit mixed up here. As I understand it the DC Lines, Watford and the North London, were LT-style +/- 4-rail, right through to Euston/Broad Street, which aided how the Bakerloo was extended out over it, and indeed the District running out to Richmond. This applied to the LNWR stock, and the replacing BR 1957 stock. Some time (1970?) the BR sections were converted to normal 3rd rail, with an earthed 4th rail where LT trains still shared the tracks.

The whole thing of stray earth currents does seem to be a "dark art" - at least to some. Although the DC lines had been there long term, when the 25Kv was extended in the 1980s over the North London 3rd rail, a whole series of such issues arose; in particular there were problems with the Victoria Line signalling, including blown LT fuses, where it passed well underneath at Highbury. One (well informed) electrical engineer referred the (doubtless also well informed) project engineers back to a classic electrical textbook, by Dover, published about 1914, which apparently describes just the issues they had run into.

The most surprising account I saw was from Stanford University, near San Francisco, in about 1970, where the biology professors had devised an experiment for measuring electrical resistance in plants, with very small microcurrents. Some interference was spoiling the experiment, they initially suspected the high-power Stanford Linear Accelerator atomic research machine, run by their colleagues, but found it was not so. Their only clue was it was likely man made, as it only happened on Mondays to Fridays. Eventually they traced it to the initial trial runs of the BART rapid transit, 1,000v DC with running rail earth return - the nearest point being about 5 miles away!
Of course there's plenty going on geologically beneath the surface in the SF region, so it might be that (physical) ground faults can cause ducting of stray earth currents over inordinate distances.
 

edwin_m

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The most surprising account I saw was from Stanford University, near San Francisco, in about 1970, where the biology professors had devised an experiment for measuring electrical resistance in plants, with very small microcurrents. Some interference was spoiling the experiment, they initially suspected the high-power Stanford Linear Accelerator atomic research machine, run by their colleagues, but found it was not so. Their only clue was it was likely man made, as it only happened on Mondays to Fridays. Eventually they traced it to the initial trial runs of the BART rapid transit, 1,000v DC with running rail earth return - the nearest point being about 5 miles away!
The BART return currents would be in the thousands of amps, so even a hundred millionth of that finding its way into the experiment would be enough to cause trouble. Possibly there was some metal utility that provided a low current return path via the vicinity of Stanford (and corroding it in the process, so the utility owner should have been grateful if they ever knew). Also quite possible the BART designers didn't understand the issue, judging by their lack of appreciation of how railway wheels are supposed to work.
 

Taunton

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Also quite possible the BART designers didn't understand the issue, judging by their lack of appreciation of how railway wheels are supposed to work.
:)

They also didn't understand a lot more. Having selected a 5'6" gauge, only then did they find none of the railway construction or maintenance equipment available from contractors in the USA was of this gauge, so they had to have it all custom made.
 

edwin_m

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:)

They also didn't understand a lot more. Having selected a 5'6" gauge, only then did they find none of the railway construction or maintenance equipment available from contractors in the USA was of this gauge, so they had to have it all custom made.
Yes, sometimes the theoretical optimum is the enemy of the practical need for compatibility. Certain members of this forum should take note...
 

Deepgreen

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The history and technical reasons are all valid, of course, but the truth is that any conversion work to other systems (third rail, 'mini-OHLE', etc ) would be hugely disruptive and would require very complex planning to allow whole lines with dedicated stock to be converted in one go (closing the whole line for a long time in the process) for a relatively small longer-term benefit. A small but significant safety benefit would be the removal of a major trip hazard in the four foot, of course. The huge logistical problems associated with whole line closures, and rolling stock and power supply re-design/conversion on such an intensively-used and strategically-vital system probably mean that we will have the current (!) system for a very long time to come.

:)

They also didn't understand a lot more. Having selected a 5'6" gauge, only then did they find none of the railway construction or maintenance equipment available from contractors in the USA was of this gauge, so they had to have it all custom made.
Yes - BART modellers use 'D'Oh' scale!
 

MarkyT

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One positive thing that conversion to 3rd rail would allow is a better, safer evacuation walkway in the 4 foot, with fewer obstructions to trip up escaping passengers and emergency personnel.
 

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One positive thing that conversion to 3rd rail would allow is a better, safer evacuation walkway in the 4 foot, with fewer obstructions to trip up escaping passengers and emergency personnel.
I’ve never heard of anyone tripping up over the fourth rail in the circumstances described as people would be walking in line with the rails rather than crossing it at a right angle.
 

MarkyT

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I’ve never heard of anyone tripping up over the fourth rail in the circumstances described as people would be walking in line with the rails rather than crossing it at a right angle.
Evacuations are very rare admittedly and will probably become rarer still when new trains have enough battery power onboard to make it to the next station during an isolation. Four rail tube tunnel rail systems ln particularly represent a very poor evacuation route and could never be made suitable for wheelchairs. With the middle rail removed, a smooth continuous walkway between the running rails might be established by contrast. With modern train control techniques that share no electrical commonality with traction return current, there's really no necessity for a fourth rail, as long as the running rail return is reinforced with extra parallel conductors, sufficiently cross-bonded to both rails at frequent intervals as required to limit the loop resistance and minimise heat loss. In a three rail DC system the running rails are mounted on electrically insulated pads and secured with insulated clip fittings, so with modern insulating materials in a dry tunnel there would be little opportunity for stray traction return current to leak out into the cast-iron tunnel segments and other surrounding metalwork; current should be confined to the rails and any parallel reinforcement conductors provided.
 

Mojo

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Evacuations are very rare admittedly and will probably become rarer still when new trains have enough battery power onboard to make it to the next station during an isolation. Four rail tube tunnel rail systems ln particularly represent a very poor evacuation route and could never be made suitable for wheelchairs. With the middle rail removed, a smooth continuous walkway between the running rails might be established by contrast. With modern train control techniques that share no electrical commonality with traction return current, there's really no necessity for a fourth rail, as long as the running rail return is reinforced with extra parallel conductors, sufficiently cross-bonded to both rails at frequent intervals as required to limit the loop resistance and minimise heat loss. In a three rail DC system the running rails are mounted on electrically insulated pads and secured with insulated clip fittings, so with modern insulating materials in a dry tunnel there would be little opportunity for stray traction return current to leak out into the cast-iron tunnel segments and other surrounding metalwork; current should be confined to the rails and any parallel reinforcement conductors provided.
I’m not quite sure about that, the 4 Foot is littered with numerous equipment depending on locations such as TBTC signal loops, tuning units, high voltage cables, point machines, stored rails and so on. Even with a customer using a wheelchair on board then they would never be able to get off by themselves in a traditional tube tunnel due to the need to descend steps from the cab to track level. If there was a need then a wide enough passage could be made even with the fourth rail being present, but you would still come across the obstructions mentioned above not to mention the difference in height between the bed and the sleepers and also not to mention the fact much of the deep tube has ballast between the sleepers. Detrainments involving customers in wheelchairs are normally handled by the use of carry chairs assisted by the Emergency Response Unit.

Also I’m not sure that a battery powered train would ever assist matters, most train to track detrainments are as a result of asset failures or other obstructions on the track rather than a problem with the train not being able to draw power.
 
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AM9

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so with modern insulating materials in a dry tunnel there would be little opportunity for stray traction return current to leak out into the cast-iron tunnel segments and other surrounding metalwork; current should be confined to the rails and any parallel reinforcement conductors provided.
Are all tube tunnels dry, (all the time)?
 

thedbdiboy

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I think that's an answer to why the 4th rail was retained at all, which is fair enough, but was wondering why the trouble was taken to drop it down to the sleepers and remove the insulators.
The 4th rail was not dropped to the sleepers immediately. Bakerloo services were cut back to Stonebridge Park in 1982 but then restored as far as Harrow & Wealdstone in 1984, which required no additional work as the 4th rail was still intact. North of Harrow & Wealdstone it was not used after 1982, so when any track work was required, there was no effort made to ensure that it was maintained at operational height. Later on, when sections were completely renewed a standard running rail laid in the four foot was utilised to retain the negative earth bonding. Nearly 40 years on it is hardly surprising that very little of the original 4th rail north of Harrow remains on pots.
 

Taunton

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Given the water ingress in some of the older stations, I'd imagine the tunnels are in an even worse state
Not even the oldest ones. Highgate, from the 1940s, appears to be the worst I know.
 

Dstock7080

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Given the water ingress in some of the older stations, I'd imagine the tunnels are in an even worse state
We had the Jubilee Line closures Finchley Road-Waterloo in 2014 due to tunnel repairs on the concrete section around Bond Street, where many rings, dating from only mid-1970s were replaced
 

321over360

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Are all tube tunnels dry, (all the time)?
Nope, was always water flowing into the track bed area of the Eastbound Central Line running lines at Mile End for as long as i can remember whenever i changed from the District to the Central i always saw a small flow of water
 

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The Eastbound tunnel between White City and Shepherds Bush seems to have a stream running under it in the four foot. Only place on the Underground I’ve ever seen a rat.
 

321over360

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The Eastbound tunnel between White City and Shepherds Bush seems to have a stream running under it in the four foot. Only place on the Underground I’ve ever seen a rat.
seen plenty of mice at several tube stations including the Central Line
 

thenorthern

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As others have said it doesn't in theory need a fourth rail but it provides a lot of benefits in regards to safety.

In an ideal world though there wouldn't be a 3rd/4th rail system anymore as the technology itself is rather obsolete and has a lot of drawbacks that overhead electrification doesn't have.
 

edwin_m

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As others have said it doesn't in theory need a fourth rail but it provides a lot of benefits in regards to safety.

In an ideal world though there wouldn't be a 3rd/4th rail system anymore as the technology itself is rather obsolete and has a lot of drawbacks that overhead electrification doesn't have.
Third rail is still the goto technology for new metro schemes, albeit using an under-contact system like DLR to improve safety, and sometimes a voltage as high as 1500V to increase efficiency. It's on the main line network where live rail systems are less suitable.
 

plugwash

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The UK doesn't really seem to believe in "pure" metros though, the only pure systems seem to be the glasgow subway and the DLR.

All of the others are hybrids, with street running tramway sections and/or sections shared with the national rail network. Other than the London underground, these hybrid systems all currently use overhead electrification. I'm pretty sure the south wales metro is also planning to use overhead electrification. Coventry very light rail seems to be planning on a system of batteries and charging stations.
 
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edwin_m

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The UK doesn't really seem to believe in "pure" metros though, the only pure systems seem to be the glasgow subway and the DLR.

All of the others are hybrids, with street running tramway sections and/or sections shared with the national rail network. Other than the London underground, these hybrid systems all currently use overhead electrification. I'm pretty sure the south wales metro is also planning to use overhead electrification. Coventry very light rail seems to be planning on a system of batteries and charging stations.
That's not unreasonable - systems that interwork with the main line need to use the same system as the main line does, or accept dual-system trains or infrastructure. The anomaly is the third rail system on the main line, originally selected by the LSWR for compatibility with the Underground I believe. I don't think there is anywhere else that does that on a main line railway, other than the Long Island (top contact) and Metro-North (bottom contact) ones in New York, which were introduced about the same time as on the Underground in London.
 

100andthirty

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There are all sorts of interesting one off systems around the world. The metre gauge St Gervais - Vallorcine line in the French Alps used 3rd rail top contact yet runs in snow prone winter quite reliably I understand.

For overhead there are two overhead systems that handle dual voltage differently. In Bilbao, Spain there is a metre gauge regional line electrified at 1500V dc and there are some urban trams electrified at 750V dc that share the same track for a short distance. The voltage is switched depending on which line is signalled to use the shared track.

In Zurich, Switzerland, there is a local line to Uetliberg - a mountain tourist spot - that is electrified at 1500V dc. It shares tracks in "downtown" Zurich with other trains that use 15kV 16 2/3 Hz ac. Here they use two separate conductor wires. The ac wire in in the conventional location above the centreline of the trains. The dc wire is offset to one side (the right in the leaving Zurich direction) with the pantograph located in a very unusual position as the photo in this link shows: https://www.bigstockphoto.com/image...liberg-railway-line-at-the-station-on-the-top
 

The DJ

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IMHO the fourth rail is not doing any harm, can be useful in extreme circumstances and would involve considerable expenditure to remove it systemwide. May the fourth be with you. Always.
 

bassmike

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I went from Old St to Moorgate on GN City line yesterday and noticed that there was fourth rail for some of the distance along the up platform, - resting on sleepers but also connected by heavy cables.
The southern had return rails in the 4 foot ont the Hayes branch and between Charlton and Woolwich.
 
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