Future of Third Rail

[tomuk]

For point 3, you would want the rail such that when touch by humans or animals that they do not get electrocuted, but still provide the power for the units operating on the rails. It should also be something that could be used with existing third rail shoes on existing EMU's, otherwise you will have just a higher cost than you would have if you had changed any third rail routes into OHLE routes potentially.

On any of the routes where third rail exists, I doubt that you will ever need any speed above 100mph, which is why I doubt that there is a need to be changing the third rail for higher speeds, other than to be changing the makeup of the third rail itself to be safer as per my comment above.

I'm not suggesting changing the design of the third rail to improve safety I think that is irrelevant, I'm suggesting upgrading it to reduce power losses i.e. reduce it resistance by using better conducting materials

 

[Pigeon]

Out of interest, just how inefficient is 3rd rail compared to OHLE when considering energy lost? Obviously I get that it has other serious limitations aside from efficiency like how much power can be drawn (considering it is a 750V system) and safety, but is it really as bad as people make out?

The "inefficiency" characteristic is essentially a more accurate statement of the same aspect as is represented in looser descriptions such as "how much power can be drawn (considering it is a 750V system)". The fundamental point is that the power loss per unit length of the same conductor, for a load of the same power, goes as the square of the current - or as the inverse of the square of the voltage, which is essentially the same thing for the same power load. So this source of power loss is (25000/750)^2 = 1111 times more significant for the lower voltage system. Eek!

Of course, the two systems are not using the same conductors. The standard contact wire for OHLE is I believe 16x16mm copper (alloy), which taking the resistivity as the same as copper at 16.8nΩ.m gives a resistance per kilometre of 1000*16.8e-9/(16e-3^2) = 65.625mΩ/km. The support wire is in parallel with this, but I don't know its size or material; assuming it's about the same we probably end up with a total of about 33mΩ/km.

Conductor rails are of steel, and the popular sizes appear to be 100 and 150 lb/yd (aargh) - so call it 50 or 75 kg/m. Using a density of 7850kg/m^3 we get cross-sectional areas of 0.0064 or 0.0096 m^2, which looks realistic; using a resistivity value of 100nΩ.m gives a resistance per kilometre of 1000*100e-9/(0.0064 or 0.0096) = 16 or 10 mΩ/km. This brings the comparison factor for losses per km down to 274 or 171. It's no surprise that at some locations a third rail system can see the voltage at the train drop by 30% or so when it starts off.

The steel-faced aluminium conductor rail they use on the Underground apparently has a resistivity of 7mΩ/km. So if this was used universally on the Southern network the resistive losses per km would now only be 120 times worse...

However, one of the much-touted advantages of 25kV is that "the feeder stations can be spaced much further apart". So although the losses per km between the feeder and the train are much lower, the number of km over which they are incurred is about 10 times as great. This means the hypothetical "Southern with LU Al rail" is only 12 times worse, and things are beginning to look a little brighter.

One obvious thing that suggests itself at this point is to have third rail with more feeder stations closer together, say every 500m or so. This is not quite as daft as it immediately sounds. You have to step the voltage down to somewhere roughly in the region of 750V at some point before it gets to the traction motors, so it's basically a question of whether you do this on the train or at the trackside, and there are quite a few advantages to doing it at the trackside. Most obviously, the train doesn't have to lug the weight of the transformer about, nor deal with keeping 25kV away from the passengers. A concrete slab in a hut at the lineside is a much more benign environment for electronics than the extremes of vibration, temperature, filth etc it has to put up with on a train, and not having to design it to suit severe weight and size constraints as well is a great help with thermal management and reliability; also with maintenance. Factors like the possibility of load sharing in busy areas or the extreme intermittency of load in more rural parts of the line can be taken advantage of to reduce the size of the individual units compared to the necessary worst-case continuous-load rating of ones on board trains themselves, and to further increase overall reliability. If you have feeder stations at a tenth of the current conventional spacing then you only have a tenth of the distance of high-loss path to send the current through.

Another obvious thing is to take the concept of separating the wear-resisting and current-carrying functions further than the steel-faced aluminium rails idea does already, and use steel rails with connections every few tens of metres to a heavy aluminium bus bar close to, but not actually part of, the track. The bus bar can then be made with a much larger cross-sectional area than if it has to pretend to be a rail, and the different thermal expansion coefficients of steel and aluminium aren't a problem any more, ditto their different electrochemical potentials. Ten times the area gives one-tenth of the losses.

The current return also needs to be taken out of the running rails and given a similarly low-resistance path ("Warning to staff: do not step on any rail"; the voltage developed on the running rail by the return current can be quite significant); in other words, a four-rail system rather than three. As well as the usual things such as keeping stray currents out of the signalling system and buried metal objects, this gives you the very great advantage of being able to double the voltage by stealth, as it were: one rail at +750V and the other at -750V gives you a 1500V supply to the train, but the exposed voltages on the track are still no more than 750V to ground; and of course double the voltage means one-quarter of the losses.

And of course you could increase the voltage to ground in any case; the reason for using 750V originally is that voltages around that range are most convenient for the technology of traction motors, and more particularly of traction motor control systems, that was available at the time, whereas now that we have power semiconductors coming out of our ears the limits of practicality are much wider. Safety wasn't without some influence, but the major factor was practicality, and the association with safety basically developed out of the observation that they got away with it. It's actually a rather bad voltage for safety, especially DC - "enough to make you grab on but not enough to throw you off". We'd be just as happy with 1kV or more now if they'd been using it all along.

The safety thing is overblown, in any case, because for some reason a few years ago third rails had the misfortune to be selected by the random spotlight of fashion as a new trendy cause to shriek about, and now everyone quotes the resulting report which used totally bent statistics to paint a lurid picture of people within 100m of railways dying like flies from third rails leaping up and jumping at them. I prefer to believe the report I found a couple of years before that which was written by people who did not have any particular axe to grind, which said that the number of railway staff falling victim to third rail and to overhead was about the same. (Which makes sense: overhead may be much more out of the way, but it's much easier to forget it's there, and it jumps gaps, and if you do get zapped by it you very much tend to stay zapped.)

Having said that, the idea of having many more feeders at closer spacing makes it possible to clobber a great deal of the safety objection in any case. At one feeder per section, or more, you simply have them only switch on when a train is in the section. Most of the time the live rail isn't.

It is often said that third rail precludes regenerative braking. This is no longer the case. It was a constraint imposed by nearly all practical rectification methods being passively commutated unidirectional devices. Now that actively commutated bidirectional devices are readily available, the ability of a feeder station to transfer power in either direction is something you more or less get for free.

(It is also often said that third rail precludes speeds over 90-100mph. This is because nobody has properly tried. It was true of pantographs before they properly tried, too - starting with the APT crew - the Shinkansen before that tried improperly, and took it out in maintenance. Still, in this country, for practical purposes as opposed to numbers-chasing, 100mph is mostly enough in any case - the important thing is to keep it up, and avoid having to slow down for speed restrictions and signal checks - and if efficiency is the concern, then with energy use going as the square of the speed (and power as the cube), increasing speed is really bad for you.)

Third rail is very lossy in the way it is done at the moment - ie. either stuff 100 years old doing what was practical then, or newer stuff imitating the bits that still are 100 years old to keep it all the same. It doesn't have to be done like that; with some assembly of elements such as those I have outlined into a coherent whole, its efficiency could be made comparable with what is currently accepted for overhead systems - but with vastly less disruption to install, no rebuilding all the overbridges and freaking out over tunnels, far less hassle finding clearance, all the work and all the maintenance conveniently at ground level, etc. etc.

 

[D365]

For point 3, you would want the rail such that when touch by humans or animals that they do not get electrocuted, but still provide the power for the units operating on the rails. It should also be something that could be used with existing third rail shoes on existing EMU's, otherwise you will have just a higher cost than you would have if you had changed any third rail routes into OHLE routes potentially.

The only way to achieve that would be to somehow have the third rail electrification linked in with signalling... nothing to do with the material of the conductor.

 

[AM9]

The only way to achieve that would be to somehow have the third rail electrification linked in with signalling... nothing to do with the material of the conductor.

and nothing to do with electrical efficiency either.

 

[Duncan-231192]

Why do you think that the third rail lines could not be converted to OHLE?

I didn't. It could, but it won't. Ever. It would be insanely expensive, insanely disruptive, and insanely difficult to accomplish. Practically every pre-1950's structure in the third rail area would need to be rebuilt, and that would be just the tip of the very large iceberg. While I admire ambition, ambition needs to be tempered with realism.

 

[HSTEd]

It's worth noting that developments in superconducting materials and cables may seriously reduce losses in DC electrification systems in the nearish future

A 1500V superconducting feeder is being installed in the Paris suburban network as a trial.

In effect, it would allow a substation to be positioned every couple hundred metres along the line. These virtual substations would allow large DC networks to share capacity and regeneration.

In theory the entire ex Southern Region network could be supported as an interconnected block with all rectifiers in parallel

 

[zwk500]

It's worth noting that developments in superconducting materials and cables may seriously reduce losses in DC electrification systems in the nearish future

Indeed but isn't the real value in verse for Medium or high voltages? I'm keeping an eye out for any news on the French 9kv trial if it happens.

In theory the entire ex Southern Region network could be supported as an interconnected block with all rectifiers in parallel

I think this is unlikely to happen across the entire network for cosy reasons, although it has a reasonable chance of happening in the inner London area with the highest power demands.

 

[Irascible]

Putting the batteries at substations means transmission losses in both directions, and perhaps also having to dump power as heat if the system has capped. Put the batteries on the trains, and have them directly charge other nearby trains if it's advantageous. Accumulators in heavier used start-stop ( or hilly ) areas with more intermittent services would work well on top. Intelligent energy profiling with suitably positioned static hardware as well as slightly more intelligent than usual trains would mean the energy stays where it's most needed ( barring wastage ) and the main grid feed is more of a topup system than a prime supply.

Plus you can run away from the power, of course.

Edit: and, also, partial electridication with third rail is operationally much easier than OHLE, because it's already doing it all the time! there's conductor rail gaps & swaps already, the equipment naturally deals with it unlike having to raise & lower a pantograph.

 

[Railsigns]

It could, but it won't. Ever. It would be insanely expensive, insanely disruptive, and insanely difficult to accomplish. Practically every pre-1950's structure in the third rail area would need to be rebuilt, and that would be just the tip of the very large iceberg.

Those are the same challenges encountered whenever a non-electrified line is electrified with OLE, and the challenges are always overcome. Third rail electrification has been replaced by OLE before, so why can't it ever happen again? I expect more conversions will take place in the future.

 

[43066]

Those are the same challenges encountered whenever a non-electrified line is electrified with OLE, and the challenges are always overcome. Third rail electrification has been replaced by OLE before, so why can't it ever happen again? I expect more conversions will take place in the future.

Because the DfT is unwilling enough to fund new electrifications, so expecting it to stump up the cost of converting a large area from one type of electrification to another is going to be a complete non starter. The southern third rail network also includes some of the most complex areas in the country, so it would doubtless be a lot harder/more expensive/more disruptive than electrifying routes outside London and the south east.

 

[Trainbike46]

And importantly, the benefits of changing 3rd rail to OHLE are rather limited, so that pushes the ratio of benefits to cost futher towards leaving it as is

 

[mr_jrt]

People do seem to keep conveniently forgetting in these discussions that equipment doesn't have infinite lifespans. There is a cost to leaving the DC electrification as it is, and you need to take that off the cost of any conversion works. That's the actual important cost differential.

And importantly, the benefits of changing 3rd rail to OHLE are rather limited, so that pushes the ratio of benefits to cost futher towards leaving it as is

Are they rather limited, though? You gain the ability draw many, many, many more amps, enabling longer and more frequent trains, for a start. You also gain the ability to use rolling stock from a much larger pool.

 

[Railsigns]

Because the DfT is unwilling enough to fund new electrifications, so expecting it to stump up the cost of converting a large area from one type of electrification to another is going to be a complete non starter.

The DfT may be unwilling at present, but 'the future' is a VERY long time. Future conversions don't necessarily have to take place over a large area.

 

[Trainbike46]

People do seem to keep conveniently forgetting in these discussions that equipment doesn't have infinite lifespans. There is a cost to leaving the DC electrification as it is, and you need to take that off the cost of any conversion works. That's the actual important cost differential.


Are they rather limited, though? You gain the ability draw many, many, many more amps, enabling longer and more frequent trains, for a start. You also gain the ability to use rolling stock from a much larger pool.

compared to electrifying a diesel only line, changing a 3rd rail line to OHLE has more limited benefits

Fair point that it is the cost differential between maintaining 3rd rail vs replacing it with OHLE that matters

 

[MarkyT]

Fair point that it is the cost differential between maintaining 3rd rail vs replacing it with OHLE that matters

There are a lot of substations typically with traditional 3rd rail and, in most mainline applications, a dedicated 33kV lineside distribution system to supply them. When that lot is due for major renewals is the opportunity to evaluate whether overhead wiring conversion is worth it, assuming you have or can obtain trains that are compatible with both systems so a staged changeover can be managed.

 

[HSTEd]

People do seem to keep conveniently forgetting in these discussions that equipment doesn't have infinite lifespans. There is a cost to leaving the DC electrification as it is, and you need to take that off the cost of any conversion works. That's the actual important cost differential.

The equipment in most real installations doesn't life expire at the same moment though. So at any given time a large amount of capital plant equipment will have to be discarded early if a conversion occurred.

And given the total disaster that is the 25kV electrification programme, I doubt any business case will be made any time soon for significant conversion

Are they rather limited, though? You gain the ability draw many, many, many more amps, enabling longer and more frequent trains, for a start.

Given that some of the most intensely operated railways in Britain are third rail, I am somewhat skeptical.

In addition future technologies are likely to reduce 25kV's power delivery advantage.
See the talk about superconductors.

You also gain the ability to use rolling stock from a much larger pool.

Stock you can't use because it's not cleared for the railway anyway.
Given that cascades are not particularly important in the modern era, I am skeptical this is at all a significant factor.

Indeed but isn't the real value in verse for Medium or high voltages? I'm keeping an eye out for any news on the French 9kv trial if it happens.

Indeed the 9kV trial is potentially very interesting.
But if superconducting cables prove a success it does raise questions about the optimum voltage for a DC overhead electrification system.

1500V or 3000V are likely to be perfectly adequate in a superconducting system and have lower clearances etc.
DC OHL does have the advantage of being able to coexist with third rail for long periods, so an overlay could be implemented without interfering with the regular system.

 

[AM9]

People do seem to keep conveniently forgetting in these discussions that equipment doesn't have infinite lifespans. There is a cost to leaving the DC electrification as it is, and you need to take that off the cost of any conversion works. That's the actual important cost differential.

Not just equipment, - structures on the railway have finite lives as well. So the overbridges and tunnels that were built in the 19th century will eventually need replacing or major reconstruction as the previous practice of patching-up has reached the end of the road. The number of bridges with insufficient OLE clearanceon 'ancient' mainlines is steadily reducing, often because the nature of the road traffic has mandated their replacement. Then there's the advances in insulation and OLE integration that enable easier installation of wiring without rebuilding (as would have been the case a few decades ago).

Piecemeal conversion of 3rd rail to OLE has progressively been eased over the last 20 years with the frequent provision (passive or actual) since ac OLE power supply provision on DC targetted EMUs was adopted by the RoSCos. If considering the current 'modern' fleet, in a quick rough estimate, there are 872 multiple units that operate on 750VDC 3rd rail track and have been built with provision for conversion to run under 25kV ac OLE. Of that total, 290 are running as dual voltage trains so do not need further work (with the exception of adding pantographs to the 707s of them which I imagine is a depot task). The remainder of the total would require a modification/retrofit programme.
With the considerable population of ready to run DV stock, it would possible to undertake a programme of incremental conversion to ac power by tackling certain sections of mainlines first where the returns would be the greatest. That might be the heaviest used sections probably with the most high speed running, e.g. BML and SWML.

 

[RobShipway]

People do seem to keep conveniently forgetting in these discussions that equipment doesn't have infinite lifespans. There is a cost to leaving the DC electrification as it is, and you need to take that off the cost of any conversion works. That's the actual important cost differential.


Are they rather limited, though? You gain the ability draw many, many, many more amps, enabling longer and more frequent trains, for a start. You also gain the ability to use rolling stock from a much larger pool.

Not just equipment, - structures on the railway have finite lives as well. So the overbridges and tunnels that were built in the 19th century will eventually need replacing or major reconstruction as the previous practice of patching-up has reached the end of the road. The number of bridges with insufficient OLE clearanceon 'ancient' mainlines is steadily reducing, often because the nature of the road traffic has mandated their replacement. Then there's the advances in insulation and OLE integration that enable easier installation of wiring without rebuilding (as would have been the case a few decades ago).

Piecemeal conversion of 3rd rail to OLE has progressively been eased over the last 20 years with the frequent provision (passive or actual) since ac OLE power supply provision on DC targetted EMUs was adopted by the RoSCos. If considering the current 'modern' fleet, in a quick rough estimate, there are 872 multiple units that operate on 750VDC 3rd rail track and have been built with provision for conversion to run under 25kV ac OLE. Of that total, 290 are running as dual voltage trains so do not need further work (with the exception of adding pantographs to the 707s of them which I imagine is a depot task). The remainder of the total would require a modification/retrofit programme.
With the considerable population of ready to run DV stock, it would possible to undertake a programme of incremental conversion to ac power by tackling certain sections of mainlines first where the returns would be the greatest. That might be the heaviest used sections probably with the most high speed running, e.g. BML and SWML.

I would say that making the change from 3rd rail to OHLE, would actually in many cases give you the ability to increase the line speed above 100mph, which I believe is the limit to which 3rd rail can go?

The other thing that I would state, as has been talked about in other threats is that any trains that you have are converted into BEMU trains (Battery Electric Multiple Units), such that if you do have structures where the OHLE cannot go through or under, then the trains for that section can revert to battery power which has been charged up by the use of the train on OHLE and other means, where power has been fed into the batteries. So, the only stumbling block really on that basis is the cost to the DFT, who correct me if I am wrong want a cut in pollution?

Yes, currently it might not be within the cost of the Government. But I believe that the DFT will have to move the goal post for diesel and petrol cars from 2030 to 2050, as believe that they cannot meet themselves the target of 2030, let alone the public being able to do so. There will need to be plans in replacing class 150. 153. 156, 158 & 159 within the next 7 years so that they can meet the target of 2030.

 

[D365]

The safety thing is overblown, in any case, because for some reason a few years ago third rails had the misfortune to be selected by the random spotlight of fashion as a new trendy cause to shriek about, and now everyone quotes the resulting report which used totally bent statistics to paint a lurid picture of people within 100m of railways dying like flies from third rails leaping up and jumping at them. I prefer to believe the report I found a couple of years before that which was written by people who did not have any particular axe to grind, which said that the number of railway staff falling victim to third rail and to overhead was about the same. (Which makes sense: overhead may be much more out of the way, but it's much easier to forget it's there, and it jumps gaps, and if you do get zapped by it you very much tend to stay zapped.)

My PTS (Personal Track Safety) course tells me otherwise - ”DC conductor rail” qualification is an entire day in itself. Albeit we work on the basis of a sensible exclusion zone of 300mm.

It is also often said that third rail precludes speeds over 90-100mph. This is because nobody has properly tried...

But we have tried. To the best of my knowledge, no other country runs third rail EMUs at any speed like ours.

Yes, currently it might not be within the cost of the Government. But I believe that the DFT will have to move the goal post for diesel and petrol cars from 2030 to 2050, as believe that they cannot meet themselves the target of 2030, let alone the public being able to do so. There will need to be plans in replacing class 150. 153. 156, 158 & 159 within the next 7 years so that they can meet the target of 2030.

Not sure how this is relevant to the topic of third rail (de)electrification.

 

[zwk500]

I would say that making the change from 3rd rail to OHLE, would actually in many cases give you the ability to increase the line speed above 100mph, which I believe is the limit to which 3rd rail can go?

Only 2 Third rail lines are currently 100mph - SWML (Byfleet to Eastleigh) and SEML (Tonbridge to Saltwood Jn) - so the benefits of this are limited. The benefits of conversion on the SWML are more about allowing XC to work over from OLE areas, and less losses on the long runs beyond Southampton.

I can see basingstoke to Weymouth (with Botley) being converted at some point, because that's where the network effects make sense for conversion. But I can't see much else ever converting.

 

[Lucan]

My PTS (Personal Track Safety) course tells me otherwise - ”DC conductor rail” qualification is an entire day in itself. Albeit we work on the basis of a sensible exclusion zone of 300mm.

Things have moved on then. I was just told (in LU) "Those are the electrified rails, don't touch them. We already knew that anyway of course.

 

[D365]

Things have moved on then. I was just told (in LU) "Those are the electrified rails, don't touch them. We already knew that anyway of course.

Oh yes - that’s still the rule. Treat conductors as live and dangerous at all times.

 

[BrianW]

Silly question perhaps but- what is the problem that de-3rd railing is thought to reduce or remove? Is death or serious injury from electrocution a greater problem than folk falling in front of a train?

On top of that, battery etc development seems to offer the desired 'gap-filling'.

 

[zwk500]

Silly question perhaps but- what is the problem that de-3rd railing is thought to reduce or remove? Is death or serious injury from electrocution a greater problem than folk falling in front of a train?

3 main problems - 1. The safety risk, 2. The electrical inefficiencies, 3. The operational problems of 2 different systems.

On top of that, battery etc development seems to offer the desired 'gap-filling'.

Indeed.

 

[BrianW]

3 main problems - 1. The safety risk, 2. The electrical inefficiencies, 3. The operational problems of 2 different systems.

Indeed.

At what cost? Of course if it were my beloved not coming home no expense would be too great. Are all platform edges to be protected?

 

[PGAT]

3 main problems - 1. The safety risk, 2. The electrical inefficiencies, 3. The operational problems of 2 different systems.

These are mostly just cons and don’t justify on their own the enormous cost of ripping out the rails and replacing them with OHLE

 

[zwk500]

At what cost? Of course if it were my beloved not coming home no expense would be too great. Are all platform edges to be protected?

These are mostly just cons and don’t justify on their own the enormous cost of ripping out the rails and replacing them with OHLE

Indeed, which is why there's not national plan to do anything about existing 3rd rail and why wholesale replacement is unlikely.

Basingstoke-Weymouth is the only section at serious threat of conversion, and that has a large amount of very specific factors that make conversion worth thinking about. Even so, I don't expect it to be converted while other deserving lines are without wires at all.

 

[A0wen]

I’d authorise the remaining diesel islands in the former southern electric region to be filled in with new third rail installation. It’s absolutely ludicrous that this isn’t allowed when the the “safety” impact is minimal (and IIRC statistically only really benefits trespassers, so tail wagging dog).

Elimination of the islands would allow an improvement of services on the east coastway line, and would remove the need for a diesel micro fleet with the associated long diesel ECS moves from Selhurst to the coast and back.

Uckfield is probably justified, but Marshlink looks ideal for a Battery unit (or battery / 3rd rail hybrid) with charging capability at Ashford.

 

[PGAT]

Uckfield is probably justified, but Marshlink looks ideal for a Battery unit (or battery / 3rd rail hybrid) with charging capability at Ashford.

They're both 25 miles of unelectrified track and see more or less similar passenger numbers. If anything, you would electrify the Marshlink line more as that opens the doors for HS1 services into Rye and Hastings.

 

[43066]

Uckfield is probably justified, but Marshlink looks ideal for a Battery unit (or battery / 3rd rail hybrid) with charging capability at Ashford.

I expect that’s what we will wait for in reality, yes. Although when that will be is anybody’s guess - the next GTR mainline fleet renewal in circa 20 years most likely (Unless Southeastern end up with battery units as part of the eventual networker fleet replacement, and the route is transferred over).

It would be interesting to know why it wasn’t electrified along with the rest of the coastway in the 1930s - low traffic levels presumably.