Does LU need the Fourth Rail?

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Lucan

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I've been reading the Island Line Upgrade thread (re : the Isle of Wight line) in the Infrastructure forum, and a remark was made that it has been quite easy to convert ex-LU stock from fourth to third rail electrification : just a matter of removing the centre shoes and re-routing some power cable.

So I wonder why the fourth rail continues to be used on LU. I understand that it was originally needed because when third rail was tried in early trials (in the Earls Court area around 1900), fuses and light bulbs blew in nearby properties due to earth leakages. Or something like that. Clearly the southern electric and other third rail lines have not had that problem, perhaps because of improved wiring standards (railway and/or domestic). So I wonder why LU still finds the fourth rail necessary, if it is. There would be a fortune in scrap steel and maintenance savings if the trains were converted at their overhauls and the centre rails then progressively removed during track renewals..
 
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Journeyman

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The main reason it's still used is because it works much better in tube tunnels with cast iron lining segments. It allows a current return path without anything returning to earth, which in iron-lined tunnels causes electrolytic corrosion.

It's obviously simpler and easier to use one electrification system, even if that system's benefits in some locations are a problem elsewhere.
 

rebmcr

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There would be a fortune in scrap steel and maintenance savings if the trains were converted at their overhauls and the centre rails then progressively removed during track renewals..
Conductor rails are usually aluminium, not steel.

Even rails that are steel, and in the open, tend to cost more to recover than their scrap value — and getting it up to the surface is even more difficult than that.
 

Dstock7080

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Conductor rails are usually aluminium, not steel.
Aluminium has only started to be used on LU for conductor rail in the last few years, to assist with the conversion to 750vDC on the sub-surface Lines, at limited locations. The vast majority remains steel.

(easy way to recognise, if the rail is resting on insulator pot and cradle, it’s steel; if rail being held down by the insulator pot it’s the lighter aluminium)
 
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100andthirty

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Aluminium has only started to be used on LU for conductor rail in the last few years, to assist with the conversion to 750vDC on the sub-surface Lines, at limited locations. The vast majority remains steel.

(easy way to recognise, if the rail is resting on insulator pot and cradle, it’s steel; if rail being held down by the insulator pot it’s the lighter aluminium)
A few points.....the aluminium/stainless steel conductor rail has also been used on the Victoria , Jubilee, and Northern lines. The rail is lighter than its steel equivalent, hence needing to be tied to the insulators. It is also much easier to handle.

Regarding corrosion, there are other effects too e.g. electrolysis of water ( converting water into hydrogen and oxygen) in sewers and water mains, a potentially explosive mix. When the decision was made to convert the sub surface railway to 750V 15 years ago, Thames Water were quite concerned as they initially assumed that this would be 750V tird rail. They phoned TfL and the switchboard found me. The TW person was much relieved when I explained.
 

edwin_m

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Track circuits on LU traditionally weren't designed to work with traction return currents, as they use a different pair of rails. This issue reduces as signalling is upgraded, as modern train detection systems are normally immune to return currents anyway.

As with so many other things on the railway, changing to something that appears more efficient would cause a lot of problems during the transition. During any conversion to third rail, either the fourth rail would have to be kept (bonded to the running rails) until the whole route could be converted and the train pickups re-wired, or the trains would need contactors to connect the negative side of the traction system to either the centre shoes or the wheels, probably operated by some kind of balise as it passed between converted and un-converted sections of a route. To avoid a major short circuit there would also need to be a dead section of 20 metres or so whenever changing between systems.
 

pdeaves

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As I understand it, another benefit of the four rail system is that the potential difference to earth is lower (+440V rather than +660V), which helps protect against some of the other problems other people have mentioned.
 

100andthirty

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As I understand it, another benefit of the four rail system is that the potential difference to earth is lower (+440V rather than +660V), which helps protect against some of the other problems other people have mentioned.
It's more complicated than this. The voltage between the rails is a nominal 630 V (recognising that voltage varies and some sections are now at 750 V). The + 420 V, - 210 V is provided by appropriate size resistors between the power rails and the running rails. This is provided to enable a form of earth fault detection, as the system monitors the relative voltages. If there is, say, a positive earth, the voltage between the current rails would still be 630 V, but the earth fault detector would see + 0 V and - 630 V, and would indicate a fault. If the fault moves from section to section then it's on a train and if the fault remains static, the fault is on the infrastructure. The trains can keep running, although the situation becomes more complex on modern trains which have their own earth fault detection.

So, suggesting that a lower potential difference to earth is a benefit can lull people into a false sense of security. +440 V DC is still a big risk if people come into contact with it.

I would add that 3rd rail supply, running rail return is not more efficient. There is quite a lot of voltage loss in steel conductors, so having both 3rd and 4th rails in more conductive aluminium/stainless steel is more efficient.
 

edwin_m

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I would add that 3rd rail supply, running rail return is not more efficient. There is quite a lot of voltage loss in steel conductors, so having both 3rd and 4th rails in more conductive aluminium/stainless steel is more efficient.
You could of course provide a parallel return cable connected to the running rails at intervals on a third rail system, which would be cheaper than a fourth rail. The fact that isn't done suggests that return resistance isn't seen as a problem.
 

100andthirty

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You could of course provide a parallel return cable connected to the running rails at intervals on a third rail system, which would be cheaper than a fourth rail. The fact that isn't done suggests that return resistance isn't seen as a problem.
I don't think one should enquire too much about the efficiency of the 3rd rail system! Since trains are generally fitted with electricity meters these days, I do believe that people are starting to question the difference between kWh consumed versus kWh input. But I'm not saying it's a big deal, though.

Much more important is that the cost to change from 4th to 3rd rail is much greater than any benefit. The responders above outlined many of the factors but probably the biggest is managing the earth currents. When the East London line was converted from 4th to 3rd rail a lot of money was spent on installing something akin to earth mats under the track. The line was closed for quite a long time. This is just not feasible on the tube. It has been extensively studied several times by different people and the result is always the same.
 

Recessio

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Much more important is that the cost to change from 4th to 3rd rail is much greater than any benefit. The responders above outlined many of the factors but probably the biggest is managing the earth currents. When the East London line was converted from 4th to 3rd rail a lot of money was spent on installing something akin to earth mats under the track. The line was closed for quite a long time. This is just not feasible on the tube. It has been extensively studied several times by different people and the result is always the same.
Do you know why was it converted to 3rd rail line, rather than just modified like Watford DC/Richmond/Wimbledon 4th rail lines so that 3rd rail LO trains could run on it?
 

100andthirty

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Do you know why was it converted to 3rd rail line, rather than just modified like Watford DC/Richmond/Wimbledon 4th rail lines so that 3rd rail LO trains could run on it?
The line was to be closed anyway for conversion to Overground operation, the track needed to be replaced and there was absolutely no requirement for the 4th rail. Thus there was the opportunity to put proper earth arrangements in place.
 

plugwash

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My understanding is that the shared sections on the Watford DC line are from a technical perspective a third rail line that has an extra rail for compatibility with LU stock. If you aren't going to run any LU stock there isn't any singificant benefit from haing the extra rail.
 

edwin_m

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My understanding is that the shared sections on the Watford DC line are from a technical perspective a third rail line that has an extra rail for compatibility with LU stock. If you aren't going to run any LU stock there isn't any singificant benefit from haing the extra rail.
I think the fourth rail was left in place on some sections where LU doesn't run any more, to avoid worsening the return current path which might have sent more of the return current through other things like AC mast foundations and local utilities.
 

HSTEd

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I can't imagine getting rid of fourth rail would save significant amounts of money.

The cost of the extra insulators and rail is not that dramatic, and it simplifies current return arrangements.
 

Recessio

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The line was to be closed anyway for conversion to Overground operation, the track needed to be replaced and there was absolutely no requirement for the 4th rail. Thus there was the opportunity to put proper earth arrangements in place.
Ah I see. Thanks
 

urbophile

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Can anyone confirm my hazy memory that the Wirral Line of Merseyrail (probably before the loop line was built in the 1970s) used the fourth rail system? Certainly it hasn't for many years.
 

Journeyman

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Can anyone confirm my hazy memory that the Wirral Line of Merseyrail (probably before the loop line was built in the 1970s) used the fourth rail system? Certainly it hasn't for many years.
I don't believe it ever has.
 

chiltern trev

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Merseyrail Liverpool to Southport was 4 rail when originally electrified.


Mersey Railway was originally 4 rail between Liverpool Central and Rock Ferry or Birkenhead.


The Wirral Line (Birkenhead to New Brighton and West Kirby) was electrified to 3 rail.
 
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Taunton

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Can anyone confirm my hazy memory that the Wirral Line of Merseyrail (probably before the loop line was built in the 1970s) used the fourth rail system? Certainly it hasn't for many years.
I understand it (partly) was. The Mersey Railway out to Birkenhead Park was 4th rail. When the Wirral line was electrified with 3rd rail in 1938 the trains now ran through there, the new (later Class 503) LMS units ran the West Kirby line, over the Mersey Railway to Liverpool, and the older Mersey units now ran out to New Brighton. Both sets of stock were fitted with switches to return current by either approach, and there was trackside equipment at Birkenhead Park to make the change. The second batch of 503 units were built in 1956, the old Mersey units were all withdrawn, and the Mersey route to Liverpool converted to normal 3rd rail.
 

contrex

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I think the fourth rail was left in place on some sections where LU doesn't run any more, to avoid worsening the return current path which might have sent more of the return current through other things like AC mast foundations and local utilities.
I believe that in sections where LU was not going to run, the centre fourth rail's insulators were taken away and it was dropped down onto the sleepers.
 

Taunton

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I believe that in sections where LU was not going to run, the centre fourth rail's insulators were taken away and it was dropped down onto the sleepers.
Indeed, after the Bakerloo service to Watford ended (1982?) the centre rail beyond Harrow & Wealdstone was dropped down to rest on the sleepers. This was readily visible from the WCML alongside. It's not apparent what advantage there was in doing this.
 

Nym

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Indeed, after the Bakerloo service to Watford ended (1982?) the centre rail beyond Harrow & Wealdstone was dropped down to rest on the sleepers. This was readily visible from the WCML alongside. It's not apparent what advantage there was in doing this.
Return impedance remaining the same for fault calculations.
 

contrex

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I believe the SW Division of the Southern Region laid lengths of rail in the four-foot to improve return path in the 1960s.
 

AM9

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I believe the SW Division of the Southern Region laid lengths of rail in the four-foot to improve return path in the 1960s.
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.
 

S&CLER

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I understand it (partly) was. The Mersey Railway out to Birkenhead Park was 4th rail. When the Wirral line was electrified with 3rd rail in 1938 the trains now ran through there, the new (later Class 503) LMS units ran the West Kirby line, over the Mersey Railway to Liverpool, and the older Mersey units now ran out to New Brighton. Both sets of stock were fitted with switches to return current by either approach, and there was trackside equipment at Birkenhead Park to make the change. The second batch of 503 units were built in 1956, the old Mersey units were all withdrawn, and the Mersey route to Liverpool converted to normal 3rd rail.
The LMS units ran to West Kirby on weekdays and the Mersey Railway units to New Brighton and Rock Ferry; on Sundays they swapped routes to maintain drivers' route knowledge.
 

Taunton

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Return impedance remaining the same for fault calculations.
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 LMS units ran to West Kirby on weekdays and the Mersey Railway units to New Brighton and Rock Ferry; on Sundays they swapped routes to maintain drivers' route knowledge.
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).
 
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edwin_m

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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.
Saves maintenance on the insulators? Also stray current is a bit of a black art, but providing a good conductor with frequent connections to earth (at every sleeper) probably encourages it to travel through the rail rather than through anything else.
 

Taunton

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Saves maintenance on the insulators? Also stray current is a bit of a black art, but providing a good conductor with frequent connections to earth (at every sleeper) probably encourages it to travel through the rail rather than through anything else.
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?
 

HSTEd

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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?

Well you need to put it on something to put it at the height the LU train will be expecting.

You have a catalogue with insulators in it, but probably don't have an entry in a catalogue for "non insulating stand for fourth rail"
 
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