It is often said that overhead electrification can be at a higher voltage than third rail. But I have heard of third-rail electrification as high as 1,500 volts, which is the same as Melbourne and Sydney, and the same as most DC overhead heavy rail electrification.

Had electrification in these cities originally been by third rail, somehow I imagine that converting to 25kv AC supplied by catenary would have been a lot easier, with lines have both third rail for older trains, and overhead for newer trains during the transition.

Could it be that overhead electrification was chosen for both Melbourne and Sydney because of level crossings?

It is often said that overhead electrification can be at a higher voltage than third rail. But I have heard of third-rail electrification as high as 1,500 volts, which is the same as Melbourne and Sydney, and the same as most DC overhead heavy rail electrification.

Had electrification in these cities originally been by third rail, somehow I imagine that converting to 25kv AC supplied by catenary would have been a lot easier, with lines have both third rail for older trains, and overhead for newer trains during the transition.

Could it be that overhead electrification was chosen for both Melbourne and Sydney because of level crossings?

Unlike Melbourne, Sydney had many level crossings eliminated before or during electrification, so that would not have been a consideration. There were always a few remaining in the original electrified area, but the majority had been closed or replaced by bridges or underpasses.

In England, 1500v DC was converted to AC, (sometimes 6.25kV initially but later 25kV) with very little actual infrastructure change. Even the trains were converted by adding a transformer.

Peter

Sydney is very hilly in places so I wonder if those gradients played a part in choosing Overhead instead of Third Rail?

You will notice when they electrified past Parramatta, they included an addition conductor wire to carry the load drawn and created by the Locomotives on regeneration.

It is often said that overhead electrification can be at a higher voltage than third rail. But I have heard of third-rail electrification as high as 1,500 volts, which is the same as Melbourne and Sydney, and the same as most DC overhead heavy rail electrification.

Had electrification in these cities originally been by third rail, somehow I imagine that converting to 25kv AC supplied by catenary would have been a lot easier, with lines have both third rail for older trains, and overhead for newer trains during the transition.

Could it be that overhead electrification was chosen for both Melbourne and Sydney because of level crossings?

In the 1920s, high voltage AC was not so convenient due to lack of later High Voltage semiconductors.
In the 1920s also, 1500VDC was a common world-wide standard, also double that namely 3000VDC.
In the 1950s High voltage AC started to become practical.
In the 1930s UK did propose 1500VDC for its main lines, but this was mostly deferred due to WWII.
By the time that UK started to electrify on a grand scale (West Coast Main Line) 25KV was starting to look more promising.

AFAIK, the highest DC voltage, 1200VDC, was on one of the Bury lines. Not sure about 1500VDC third rail.

A limiting factor for Third Rail is the danger of the live rail, much less than for Overhead wires. This depends a bit whether the third rail is top contact, side contact or bottom contact. This limits the voltage.

You've ignored the extensive German/Austrian/Swiss 15kV 16.7Hz electrified railway network, mostly constructed in the early-mid 20th Century. Traction motor technology at the time worked much better with 16.7Hz AC than with the 50Hz mains AC used in Europe, and AC allowed for higher transmission voltages compared to DC. 25kVAC was very much a post-WW2 technology as traction motor and rectification technology had caught up by then.

1500VDC was a more established technology in the US and UK when the VR was considering railway electrification, however. NSW and Victoria managed to settle on 1500VDC as a common standard although the prospect of electrifying the entire Melbourne-Sydney mainline at 1500VDC was preposterous then and even more so today.

LX are manageable with 3rd rail but does add some complexities. You can see photos in Google.

Overhead even 1500VDC I believe is inherently cheaper and more practical than 3rd rail, but 3rd rail was at least easier in early tunnels and visually O/H is less popular and 3rd rail probably has issues with heavy trains. hills are not an issue as UG tunnels are often steap as well. In Sydney the steepest sections I believe are in the city tunnels

My understanding for the reason behind 1500VDC was that the two traction motors in series in each bogie was this voltage, or maybe it was in parallel.

Conversion of Sydney to 25kVAC is extremely complex and I'm sure it has been discussed many times in the Sydney Trains Capital Projects office (assume they have) many times over years. I'm sure the stumbling block is the legacy rolling stock and inter connection of lines in the city and into North Shore is just a starter. Probably looking at a 30-40year project that needs continuous funding and for what operational benefit?

It is often said that overhead electrification can be at a higher voltage than third rail. But I have heard of third-rail electrification as high as 1,500 volts, which is the same as Melbourne and Sydney, and the same as most DC overhead heavy rail electrification.

Had electrification in these cities originally been by third rail, somehow I imagine that converting to 25kv AC supplied by catenary would have been a lot easier, with lines have both third rail for older trains, and overhead for newer trains during the transition.

Could it be that overhead electrification was chosen for both Melbourne and Sydney because of level crossings?

In the 1920s, high voltage AC was not so convenient due to lack of later High Voltage semiconductors.
In the 1920s also, 1500VDC was a common world-wide standard, also double that namely 3000VDC.
In the 1950s High voltage AC started to become practical.
In the 1930s UK did propose 1500VDC for its main lines, but this was mostly deferred due to WWII.
By the time that UK started to electrify on a grand scale (West Coast Main Line) 25KV was starting to look more promising.

AFAIK, the highest DC voltage, 1200VDC, was on one of the Bury lines. Not sure about 1500VDC third rail.

In England, the Woodhead Line (Manchester-Sheffield-Wath) and the line from London Liverpool St to Shenfield and Southend Victoria were electrified using 1500V DC overhead, the standard preferred by the LNER.

1500V DC overhead is still used by the Tyne and Wear Metro, a heavy metro service partially running on Network Rail metals.

The Bury line which had the 1200V side contact third rail was previously electrified using 3500V DC overhead.

In 1926 the senior engineers of the NSWGR presented a series of papers on the Sydney electrification to the Institution of Engineers, Australia. These papers were republished by the ARHS in 1987.

The Chief Electrical Engineer of the NSW, WH Myers, explicitly addressed the selection of 1500V DC overhead in Sydney in his paper.

He started by noting that the VR had tendered for both DC and AC systems; and that DC was cheaper. And he then went on to say that DC was better for heavy suburban transit. In 1922, the Conference of Railway Commissioners agreed with a recommendation from the Chief Electrical Engineers of the VR and NSW that every effort should be made to avoid the introduction of electrification that was not standard with the Victorian 1500V system.

However, NSW did consider 3rd rail systems. He said:

"The advantages of the third rail system are many, and to those how have had experience in the construction and maintenance of overhead conductors for such work, the relative mechanical simplicity of the former system seems very alluring. Moreover, experience on the 600-volt direct-current third-rail constructions of the underground systems in New York, London, and Paris is such as to prove that a reliable and satisfactory service can be given with that means of current collection, and at that voltage. Nevertheless, there was every reason to believe, when the decision was finally reached to adopt the overhead construction, that, at the higher voltage, the effects of inevitable arcing and faults on the third rail system would be much more disastrous from the point of view of reliable operation; this applies particularly with modern multiple unit trains in which no train "bus" cables are used. Moreover, the degree of safety to the staff and to the public, attainable by the overhead construction, could not be approached by any system of protected live rail, particularly in complicated railway yards, sidings, etc. The desirability of providing for long distance traffic in the future was also a determining factor in this decision, as a uniform system of overhead on such a service provides, also, for uniformity in the rolling stock collecting devices; a third rail conductor extended into the country lines was not considered feasible."

I did note that 1,500 volts can be supplied by third rail, but overhead electrification avoids having those big gaps at level crossings. The Tyne and Wear Metro is a segregated light rail, not heavy rail.

In fact, this page goes into DC supplied by a third-rail versus high voltage A.C supplied by overhead wire and even gives an example of 1,500 volt third rail.

I did note that 1,500 volts can be supplied by third rail, but overhead electrification avoids having those big gaps at level crossings. The Tyne and Wear Metro is a segregated light rail, not heavy rail.

In fact, this page goes into DC supplied by a third-rail versus high voltage A.C supplied by overhead wire and even gives an example of 1,500 volt third rail.

1500VDC exist in china and I think more modern systems. Skin effect makes LV AC systems problematic but maybe not today.

4th rail systems are older and tend to be less than 700VDC.

UK is probably a electric supply gunzels wet dream with so many different power supplies and often mixed on same track.

Skin effect makes AC unsuitable for third-rail regardless. Live rails have larger cross-sectional areas than the skin depth of mains frequency current.

Note that the highest third rail voltage is the same as most D.C electrified heavy rail with overhead wires which makes me wonder what advantage DC supplied by overhead wire would have over the same type of current supplied by a third rail, other than not needing any gas, the gaps at level crossings being especially long.

Why have 1,500 volts supplied by an overhead wire on off-street rail if the same electrical standard could be supplied by third rail?

• Water, in both liquid and solid forms.
• Safety for track workers and passengers evacuated in an emergency.
• Cost of maintenance.

There you go Myrtone. Now can you please tell us about what benefits third rail has that are so great as to override those issues.

I think the DC versus AC issue relates to the fact the only viable electric motor for traction in the early 1900's was a series DC motor, due to its high torque at low speed. AC motors have only become realistic since the development of modern electronics. The other issue was the lack of a rectifier, so if AC was used apart from requiring transformers in each vehicle, the AC would need to be converted to DC using rotary convertors. (A rotary convertor is essentially a motor generator with slip rings and a commutator). This versus a very simple system consisting of resistor starters and then when up to speed the DC traction motors are directly connected to the DC supply. So the motor vehicles are quite simple machines. The Victorian system used large rotary convertors in the substations to develop the DC. This is also preferable to many small ones in each motor car. This is also the reason 25 HZ was chosen for the main supply. No one at that time could build a commutator that would not fly apart due to centripetal forces at a 50 HZ supply. Hence the European 16 2/3 supply frequency.

• Water, in both liquid and solid forms.
• Safety for track workers and passengers evacuated in an emergency.
• Cost of maintenance.

There you go Myrtone. Now can you please tell us about what benefits third rail has that are so great as to override those issues.

All this applies to systems electrified at a tramway voltage (such as 600 volts) but off street railway electrification at such a voltage tends to be third rail.

And what about cost of maintainence?

UK is probably a electric supply gunzels wet dream with so many different power supplies and often mixed on same track.

It is indeed fascinating - London to Brighton still being 3rd rail is probably the longest working example of third rail (cross-country). As I understand Southern ordered the new rolling-stock (Electrostars?) with the capacity to go to overhead but there's no sign of it being converted any time soon.

There's plenty of examples to be seen on YouTube of the third rail arcing badly in the snow and the wet; at least up high its less of a hazard to anyone who might wander onto the tracks for any reason.

• Water, in both liquid and solid forms.
• Safety for track workers and passengers evacuated in an emergency.
• Cost of maintenance.

There you go Myrtone. Now can you please tell us about what benefits third rail has that are so great as to override those issues.

All this applies to systems electrified at a tramway voltage (such as 600 volts) but off street railway electrification at such a voltage tends to be third rail.

And what about cost of maintainence?

Maybe everyone running those voltages from overhead lines has already taken advantage of the simple ability to upgrade to 1.5kV or 3kV DC by just replacing transformers. Or they've already upgraded to AC, like the lines previously electrified by the LNER and BR Eastern Region did.  

I've listed three simple advantages of overhead electrification, in addition to those already mentioned upthread about level crossings and major junctions. Can you now either concede the arguments stack up, or just answer the question please: What are the advantages of third rail?

Let's pretend that you are a salesman at Third Rail Electrification Pty Ltd who has come into my office to pitch your product. Tell me why I would want to use your product if I was electrifying a suburban railway for the first time. If you just dodge the question and give me various versions of “that's what other railways do” without actually giving any reasons, I'm going to politely thank you for your time and get on the phone to the 25kV overhead salesman who I had in the office earlier. You're going to be lining up at Centrelink because your boss can't afford to pay a salesman who doesn't try to sell the product.

By the way, I did ask the 25kV salesman about what other railways did after he had given the presentation about his system. He mentioned that the UK has a policy of allowing no further extension of the third rail electrification system. So it looks like third rail doesn't even stack up on the “that's what other railways do” front, right? Are there any railways anywhere that are still installing new third rail these days?

UK is probably a electric supply gunzels wet dream with so many different power supplies and often mixed on same track.

It is indeed fascinating - London to Brighton still being 3rd rail is probably the longest working example of third rail (cross-country). As I understand Southern ordered the new rolling-stock (Electrostars?) with the capacity to go to overhead but there's no sign of it being converted any time soon.

London-Brighton is about 82km, and is actually the shortest route from London to the south coast. Other third rail routes in the former Southern Railway area longer than that include:
South Western Mainline (London Waterloo to Weymouth) at 272km
Chatham Mainline (London Victoria to Dover Priory via Chatham) at 125km
South Eastern Mainline (London Charing Cross to Dover Priory via Ashford) at 123km
Portsmouth Direct Line (London Waterloo to Portsmouth Harbour) at 105km
West Coastway (Southampton to Brighton) at 100km

If it were not for the internal malaise at Network Rail, they would be looking at extending 25kV electrification from the down to the south coast at Southampton for freight purposes. That will be back on the table once they sort out their project management issues.

The UK has a policy that all new third rail stock has to be either dual-voltage or have the capacity for the 25kV kit to be installed later. Southern's most recent Electrostar order (Class 387) and some of their older Class 377 Electrostars are dual-voltage, but most of their Class 377 fleet are just third rail. Southern use the dual-voltage Class 377s to run cross-London services to Milton Keynes (using 25kV on the West London Line and West Coast Main Line) and the Class 387 fleet was specified as dual-voltage as there is a high chance of it being transferred to another operator.

• Water, in both liquid and solid forms.
• Safety for track workers and passengers evacuated in an emergency.
• Cost of maintenance.

There you go Myrtone. Now can you please tell us about what benefits third rail has that are so great as to override those issues.

Tunnels and structures can be smaller.  Look at the the deep level tube lines vs the sub-surface lines vs mainlines in london for example.

Maybe everyone running those voltages from overhead lines has already taken advantage of the simple ability to upgrade to 1.5kV or 3kV DC by just replacing transformers. Or they've already upgraded to AC, like the lines previously electrified by the LNER and BR Eastern Region did.

Not here in Melbourne, nor in Sydney. It is said that converting either to A.C is more complicated than it's worth.

I've listed three simple advantages of overhead electrification, in addition to those already mentioned upthread about level crossings and major junctions. Can you now either concede the arguments stack up, or just answer the question please: What are the advantages of third rail?

First of all, third rail is cheaper than overhead electrification, it doesn't need poles can can be installed with much less civil engineering work.
I asked why 1,500 volts is supplied by overhead wiring if the same electrical standard could be supplied by a third-rail and your answer leaves unexplained why off street electrification less than 1kv tends to be with third rail.

Let's pretend that you are a salesman at Third Rail Electrification Pty Ltd who has come into my office to pitch your product. Tell me why I would want to use your product if I was electrifying a suburban railway for the first time. If you just dodge the question and give me various versions of “that's what other railways do” without actually giving any reasons, I'm going to politely thank you for your time and get on the phone to the 25kV overhead salesman who I had in the office earlier. You're going to be lining up at Centrelink because your boss can't afford to pay a salesman who doesn't try to sell the product.

Trouble is you are comparing third-rail electrification (always DC and at a voltage of at most 1,500) to AC overhead electrification at a higher voltage. How about comparing DC a given voltage supplied by third-rail to the same electrical standard supplied by an overhead wire?

By the way, I did ask the 25kV salesman about what other railways did after he had given the presentation about his system. He mentioned that the UK has a policy of allowing no further extension of the third rail electrification system. So it looks like third rail doesn't even stack up on the “that's what other railways do” front, right? Are there any railways anywhere that are still installing new third rail these days?

Much DC electrified off-street rail (except for 3kv systems) has third-rail as means of supplying power to trains, yes even on newer metro systems.

No further comment on the situation in England at the moment.

Without level crossings, it seems likely that third-rail electrification is at an advantage for off-street rail over the same electrical standard supplied by an overhead wire.

Some third-rail electrified lines (but not in England) have insulated third rail. There is insulated top contact, like the English standard, but with insulation. And then there are side-contact and bottom contact systems, both are always insulated. Insulation protects from rain and snow and improves safety for people on tracks.

1500VDC third rail is rare and the quick look I had seemed to be China based. I think this would seem to indicate there are significant complexities in 1500VDC 3rd rail. Not sure if these lines are just tunnel or surface running.

Even new commuter networks like Dubai Metro went for sub 1kV 3rd rail.  

Looking at the modern 3rd rail systems installation with extensive insulation, it doesn't look neither simple nor cheap.

Additional how does freight work with modern 3rd rail for clearances etc?

For me today the choice of power supply fora greenfield project would be in preference order such as
1) 25kV OH or other high voltage AC
2) 1.5 to 3kVDC OH
3) 3rd rail sub 1kV for tunnel based to allow smaller bore or where OH is visually unacceptable.

Considering the Sydney Metro is not interchangeable with Sydney trains I'm a bit surprised they didn't go 25kV. Perhaps the cost of conetting existing sections just didn't stack up?

It is often said that overhead electrification can be at a higher voltage than third rail. But I have heard of third-rail electrification as high as 1,500 volts, which is the same as Melbourne and Sydney, and the same as most DC overhead heavy rail electrification.

Had electrification in these cities originally been by third rail, somehow I imagine that converting to 25kv AC supplied by catenary would have been a lot easier, with lines have both third rail for older trains, and overhead for newer trains during the transition.

Could it be that overhead electrification was chosen for both Melbourne and Sydney because of level crossings?

The 1500v DC overhead was chosen as it was the best solution at the time when the Melbourne and Sydney systems were introduced.


Nothing to do with level crossings.


If those systems had been electrified with third rail then the clearances in tunnels and to overbridges would have been less than what now exists.


So converting to overhead supply would have needed raising of all overhead structures to provide the necessary clearances.


To convert from 1500v DC to 25,000v AC would be difficult as the "static clearance" for the former is, or was back when I was involved in these matters, 150mm, whereas for the AC system it is 300mm.

1500VDC third rail is rare and the quick look I had seemed to be China based. I think this would seem to indicate there are significant complexities in 1500VDC 3rd rail. Not sure if these lines are just tunnel or surface running.

I believe most 1,500 volt electrification is on lines with level crossings.

Even new commuter networks like Dubai Metro went for sub 1kV 3rd rail.

I wonder if it has to do with the length of these lines, combined with the cross sectional area.

Looking at the modern 3rd rail systems installation with extensive insulation, it doesn't look neither simple nor cheap.

But surely still simpler a cheaper than overhead wires because it still doesn't need gantries or auto-tensioning - usually with weights and ratchets.

I have no idea what freight has to do with it.

For me today the choice of power supply fora greenfield project would be in preference order such as
1) 25kV OH or other high voltage AC
2) 1.5 to 3kVDC OH
3) 3rd rail sub 1kV for tunnel based to allow smaller bore or where OH is visually unacceptable.

I believe that 25kV is only economical for large networks with long line length. Third-rail seems to be the best default for off-street electrification at voltages that don't demand more clearance tha

Considering the Sydney Metro is not interchangeable with Sydney trains I'm a bit surprised they didn't go 25kV. Perhaps the cost of conetting existing sections just didn't stack up?

It's only a single line, and maybe not long enough for 25kV.

@br30453

The 1500v DC overhead was chosen as it was the best solution at the time when the Melbourne and Sydney systems were introduced.

Did 1,500 volt third-rail exist at the time?

Nothing to do with level crossings.

But I did note that third-rail has gaps at level crossings, while overhead doesn't so how can you claim that?

If those systems had been electrified with third rail then the clearances in tunnels and to over-bridges would have been less than what now exists.

The clearances would not have been increased with third rail electrification, unless it would make way for taller trains.

So converting to overhead supply would have needed raising of all overhead structures to provide the necessary clearances.

Adding overhead supply always needs that, whether there was previously a third-rail or not.

To convert from 1500v DC to 25,000v AC would be difficult as the "static clearance" for the former is, or was back when I was involved in these matters, 150mm, whereas for the AC system it is 300mm.

Do you mean converting between two standards supplied by an overhead wire?

As noted, converting a large electrified network between one standard supplied by third-rail to another supplied by an overhead wire is actually simpler than converting between different overhead standards, even though the former needs more civil engineering work.
This is because the network, whole or in part, can have both electrical standards at the same time, as opposed to some part only being electrified with one standard and other parts electrified with only the other.

Myrtone, scroll up. In a previous post I quoted the words of the Chief Electrical Engineer of the NSWGR specifically addressing the issues of 1) why they chose DC instead of AC, and 2) why they chose overhead distribution rather than third rail.

If the NW Metro was to be built with 25kV AC OH, they would only need one feed in sub at Epping and likely a smaller one at depo at the end of the line feeding back a station or two.

With the City Metro, a sub at North Sydney, then near the junction of the Sydnemham would probably do it.

1500VDC third rail is rare and the quick look I had seemed to be China based. I think this would seem to indicate there are significant complexities in 1500VDC 3rd rail. Not sure if these lines are just tunnel or surface running.

I believe most 1,500 volt electrification is on lines with level crossings.

The Guangzhou Metro lines with 1500V third rail do not have any level crossings. They all have short sections that are elevated with the rest underground.

It is interesting to note that most of the more recently opened lines of the Guangzhou Metro system have all used 1500V overhead.

This shift away from 1500V third race would mirror the poor experience that the French had with 1500V third rail on the Culoz-Modane route. This line was electrified in 1925 and then replaced with 1500V DC overhead in 1976. None of the other connecting lines that were electrified later used the same standard.

Worldwide, this means that the majority of route kilometres electrified with 1500V DC third rail have had the system later replaced with overhead. There's always the possibility that the Guangzhou Metro third rail lines might follow when the infrastructure comes up for renewal, going on the poor record that China has with getting rail projects right the first time.

Looking at the modern 3rd rail systems installation with extensive insulation, it doesn't look neither simple nor cheap.

But surely still simpler a cheaper than overhead wires because it still doesn't need gantries or auto-tensioning - usually with weights and ratchets.

That stuff looks a little complicated, but is really quite easy to install.

You dig holes every 50-60 metres, put the masts/gantries in and set the registration arms to the correct width. Really quite an easy job, no need to maintain tight tolerances along every single metre of the line and install large quantities of insulation, and it doesn't get in the way of maintaining the rails, sleepers or ballast.

The 1500v DC overhead was chosen as it was the best solution at the time when the Melbourne and Sydney systems were introduced.

Did 1,500 volt third-rail exist at the time?

It had been installed on the Modane-Culoz line in France, before the overhead system was installed in Sydney.

As explained already,

As noted, converting a large electrified network between one standard supplied by third-rail to another supplied by an overhead wire is actually simpler than converting between different overhead standards, even though the former needs more civil engineering work.
This is because the network, whole or in part, can have both electrical standards at the same time, as opposed to some part only being electrified with one standard and other parts electrified with only the other.

Easily solved with the use of a fleet of dual-voltage EMUs and a couple of diesel locos to haul them across each 'gap' which is being worked on.

If you're running a railway and you have electrical infrastructure which is approaching the time of requiring major renewal work, all options would be on the table at the point of planning the project. Speaking of all options being on the table…

Let's pretend that you are a salesman at Third Rail Electrification Pty Ltd who has come into my office to pitch your product. Tell me why I would want to use your product if I was electrifying a suburban railway for the first time. If you just dodge the question and give me various versions of “that's what other railways do” without actually giving any reasons, I'm going to politely thank you for your time and get on the phone to the 25kV overhead salesman who I had in the office earlier. You're going to be lining up at Centrelink because your boss can't afford to pay a salesman who doesn't try to sell the product.

Trouble is you are comparing third-rail electrification (always DC and at a voltage of at most 1,500) to AC overhead electrification at a higher voltage. How about comparing DC a given voltage supplied by third-rail to the same electrical standard supplied by an overhead wire?

If it's my railway that I want electrified, I will be the one to decide which options will be excluded from consideration. Bidders don't get to decide who they feel happy competing against, if that means you miss out on the contract then you should perhaps consider improving your product to the point where it is the best option available.

The Guangzhou Metro lines with 1500V third rail do not have any level crossings. They all have short sections that are elevated with the rest underground.

Maybe the absence of level crossings on those metro lines mean that third-rail could be used. Someone pointed out that 1,500 volts (this time note the comma) DC is more commonly supplied by an overhead wire than a third rail and I naturally suspected that this is because most (not all) 1,500 volt electrified networks have level crossings and overhead wires don't normally need to be gapped, at level crossings or elsewhere.

This shift away from 1500V third race would mirror the poor experience that the French had with 1500V third rail on the Culoz-Modane route. This line was electrified in 1925 and then replaced with 1500V DC overhead in 1976. None of the other connecting lines that were electrified later used the same standard.

It's not clear whether this route has level crossings, but it had overhead at stations right from the start. I wonder if the conversion to overhead had something to do with conformity with other (connecting) lines. Maybe other lines in France weren't electrified with this standard because of level crossings.
It's also not clear what third rail geometry was used in this case, bottom contact may allow for higher voltages at most.

That stuff looks a little complicated, but is really quite easy to install.

You dig holes every 50-60 metres, put the masts/gantries in and set the registration arms to the correct width. Really quite an easy job, no need to maintain tight tolerances along every single metre of the line and install large quantities of insulation, and it doesn't get in the way of maintaining the rails, sleepers or ballast.

But that doesn't mean third rail isn't even easier, it can be installed without civil engineering work needed for overhead wires.

As explained already, .... Easily solved with the use of a fleet of dual-voltage EMUs and a couple of diesel locos to haul them across each 'gap' which is being worked on.

And yet converting between different standards both supplied by an overhead wire is often more complicated than it's worth, as in Melbourne and Sydney.

When converting from one electrical standard supplied by a third rail to another supplied by an overhead wire, the all or part of the network will have the two standards co-exist as pictured below.

With no need for dual voltage locomotives or motor coaches, nor gaps in the power supply.

No comment on the rest.