What’s more efficient: overhead or 3rd rail electrification?

[Taunton]

The oddball frequencies arise from the standard of measuring electrical AC in Cycles per Second (Hertz), whereas the mechanical generation plant is more commonly measured in Revolutions Per Minute, or RPM. Thus 50Hz is 3,000 RPM, 16.6Hz is 1,000 RPM, etc. The public electricity supply standardised on 50Hz in the UK, and 60Hz in the USA. You could convert in the days before power electronics, but it not only required bulky and inefficient local rotary converters, but was a real challenge outside the power station to keep these all synchronised for speed with one another.

The USA have converted some of their 25Hz AC to 60Hz, so they can readily use the public electricity supply rather than their own independent system. I'm not sure what changes were required to the rolling stock to achieve this. The AC voltage stayed the same. The USA High-V distribution lines are carried on high extensions to the lineside masts, at road overbridges they are carried up extra high right over the top of everything.

Indeed - it's easier to have a multi-voltage train and a short gap (or overlap) between systems, rather than trying to get 2 systems to co-exist over a significant length. Hence why the 3KV DC - 25KV AC conversion is interesting, as it means that the existing DC area must be readied for AC while still handling DC before it's changed over.

The interface between the French 25kV AC and Italian 3kV DC at Ventimiglia, on the Riviera, is interesting. Most modern French stock is dual AC/DC, so about a mile before the station there is a transition to French 1,5kV DC, seamlessly on the move. This runs through the station, where Italian DC locomotives equally circulate on half voltage. A mile on the Italian side there's a further transition to full voltage.

The lineside changeover plant there always reminds me of that on the London Underground at the likes of Queens Park or Putney Bridge, where their trains again seamlessly run from a fourth rail +420V/-210V to a +630V/0 system, with the 4th rail (for the Underground trains pickups) bonded to the running rails for the return current, which the main line trains use.

I would consider that the existence of precisely 1 third rail network of more than 1000 miles on the entire planet answers the question pretty thoroughly

How many track miles (not route miles) are there on the London Underground or the New York Subway? And how many passengers per day do they handle compared to the WCML?

 

[edwin_m]

How many track miles (not route miles) are there on the London Underground or the New York Subway? And how many passengers per day do they handle compared to the WCML?

That isn't really relevant to the question of which is better for main line railways. I think we can agree there are lots of reasons why third rail can be a better choice for metros.

 

[Sorcerer]

They were heavily derated on 3rd rail compared to performance on OHLE. Over many sections of expected highest demand, the supply was upgraded to 'high current' standards. The TP (track paralleling) huts that normally alternate with substations along the line were converted to be full supply substations as well with new HV connections. With the magnitude of current, the resistive losses in the conductors and running rail return would definitely increase heating effects.

Guess it just goes to show that overhead lines really are the better form of electrification then. More specifically 25k 50/60Hz for new electrification or even 15kv 16.7Hz if you're connecting to an existing legacy network.

How many track miles (not route miles) are there on the London Underground or the New York Subway? And how many passengers per day do they handle compared to the WCML?

In the case of London Underground it's about 402km (250mi) of track, and the New York City Subway about 1070km (665mi) of track. Both systems handle about 3.5 million passengers per day. The only mainline third rail systems outside the UK is the Hudson and Harlem part of the Metro-North Network in the US, and the Mitre, Sarimento and Urquiza lines in Buenos Aires, Argentina. I could be missing a few but it's generally not the norm.

I think we can agree there are lots of reasons why third rail can be a better choice for metros.

I think a more appropriate statement would be that metros are the best case scenario where you can argue for a third rail system because tunnel sizes matter more than the number of substations. Whether it's a better choice in general is a different debate, and New Delhi and Hong Kong still use overhead lines for their rapid transit networks.

 

[Bald Rick]

How many track miles (not route miles) are there on the London Underground or the New York Subway?

To add to others’ answers, the NY subway has 850 miles of track, albeit nearly 200 miles of it is not in ‘revenue service’, presumabky in depots, little used curves, chords and access lines.

 

[edwin_m]

In the case of London Underground it's about 402km (250mi) of track, and the New York City Subway about 1070km (665mi) of track. Both systems handle about 3.5 million passengers per day. Outside that the only mainline third rail systems outside the UK is the Hudson and Harlem part of the Metro-North Network in the US, and the Mitre, Sarimento and Urquiza lines in Buenos Aires, Argentina. I could be missing a few but it's generally not the norm.

Neither the Underground nor the New York Subway are main line railways. If you include them you can also include dozens of metros worldwide that use third rail. There are indeed very few main line railways using third rail, which is the key point here, but the Long Island Railroad is another one (different network and different third rail type from Metro-North), and after that I can only think of the one through Chamonix to the Swiss border.

 

[Sorcerer]

Neither the Underground nor the New York Subway are main line railways. If you include them you can also include dozens of metros worldwide that use third rail. There are indeed very few main line railways using third rail, which is the key point here, but the Long Island Railroad is another one (different network and different third rail type from Metro-North), and after that I can only think of the one through Chamonix to the Swiss border.

I didn't intend to suggest the Underground or Subway were main line railways, but my wording may have lead to that confusion so I have duly corrected myself. I wasn't aware of the Chamonix line, but if you refer to the one running from Saint-Gervais to Vallorcine I am lead to wonder whether that could be classed as a main line railway since it's only single-track and relatively short. Interestingly it's third rail voltage of 850 is higher than UK standards.

 

[MarkyT]

I wasn't aware of the Chamonix line, but if you refer to the one running from Saint-Gervais to Vallorcine I am lead to wonder whether that could be classed as a main line railway since it's only single-track and relatively short. Interestingly it's third rail voltage of 850 is higher than UK standards.

It is also narrow gauge, like the little yellow train in the Pyrenees, also 3rd rail. In addition to the 3rd rail only trains, there's a subfleet co-owned with the Swiss Martigny–Châtelard Railway also equipped with a pantograph and rack for running through to Martigny, a service known as the Mont Blanc Express. That rolling stock is being renewed with the cross-border capable fleet size increasing using a custom design from Stadler.

 

[Elecman]

I must have imagined all the incidents of rail damage casued by bridging between con rail and running rail. Including this week.

Me too !!

== Doublepost prevention - post automatically merged: 30 May 2025 ==

I'd be surprised if they were caused by something as low impedance as a shopping trolley. Don't know any details though.

A drinks can vapourising itself and striking an arc could do meaningful damage.

Shopping trolleys, scooters and even bicycles have all caused damage to the .3rd rail and running rails where they shorted the rails together

 

[Jimini]

Sounds like a job for Ian Faletto to me

 

[GraphitProjekt]

because tunnel sizes matter more than the number of substations.

Another thing that can play into it is whether the number of trains you have running outnumbers the number of substations you have. A metro is a railway with comparatively short distances but quite high train frequency (so lots of trains), therefore you're going to be maintaining fewer transformer/rectifiers by having them in substations and feeding trains DC directly. The 25kV AC system is basically just about moving the transformer/rectifier function onto the trains and building fewer substations as as result. Specifically for third rail, I have heard it said that there are marginal gains to be had for acceleration of multiple train at once in that the contact area is much wider mostly because a third rail train has many shoes in contact rather than just the one pantograph. (That being said, DC OLE railways tend to increase the number of pantographs on EMUs)

 

[Nicholas Lewis]

So we already know that overhead lines can provide higher voltages than third rail which means traction can draw more power without an increase in amperes, but recently I heard elsewhere that when the old Eurostar 373s were taking the routes into Waterloo that they almost melted the third rail conductor because of the amount of power they needed. Can anyone confirm this assertion?

They never melted the conductor rail that would take serious amounts of power. The trackside cabling was a vulnerable area and we doubled up every cable to 2 x 1000 sqmm cables to provide resilience but there were odd occasions where the lugs burnt off.

For clarity a class 373 was current capped at 6800 Amps at 750V DC. Had it been allowed its full installed rating that would have needed 24,500 Amps !!

 

[Sorcerer]

They never melted the conductor rail that would take serious amounts of power. The trackside cabling was a vulnerable area and we doubled up every cable to 2 x 1000 sqmm cables to provide resilience but there were odd occasions where the lugs burnt off.

For clarity a class 373 was current capped at 6800 Amps at 750V DC. Had it been allowed its full installed rating that would have needed 24,500 Amps !!

So while it wouldn't have literally melted the third rail it does still sound like a 373 was a very power-hungry monster of a machine to say the least. Evidently they were wasted on the third rail system which is natural given what they were designed for.

 

[Sun Chariot]

So while it wouldn't have literally melted the third rail it does still sound like a 373 was a very power-hungry monster of a machine to say the least. Evidently they were wasted on the third rail system which is natural given what they were designed for.

Yes and bear in mind the "pedestrian speed" routing out of Waterloo, was always an interim solution until HS1.

 

[GraphitProjekt]

Double headed class 90s used to register as fault current on the WCML until they purposefully regeared them to draw less horsepower.

 

[D365]

Double headed class 90s used to register as fault current on the WCML until they purposefully regeared them to draw less horsepower.

Regeared, or the isolation of two pairs of traction motors for double heading?

 

[Richard Scott]

Regeared, or the isolation of two pairs of traction motors for double heading?

I believe it was one motor per loco that was isolated.

 

[D365]

I believe it was one motor per loco that was isolated.

D’oh. Obviously. They are Bo-Bo locos.

 

[hwl]

Double headed class 90s used to register as fault current on the WCML until they purposefully regeared them to draw less horsepower.

Regeared, or the isolation of two pairs of traction motors for double heading?

I believe it was one motor per loco that was isolated.

That was old way of doing things, the modern solution is to limit the max power per loco in multi, there was no regearing involved.

 

[Nicholas Lewis]

So while it wouldn't have literally melted the third rail it does still sound like a 373 was a very power-hungry monster of a machine to say the least. Evidently they were wasted on the third rail system which is natural given what they were designed for.

Clearly it need the installed power to reach 300kmh but they could just about make 160kmh on the third rail although the hotel load for a 400m train short changed the traction system as well.

 

[edwin_m]

That was old way of doing things, the modern solution is to limit the max power per loco in multi

Limiting power has the advantage that the loco still has its maximum tractive effort at low speeds, which is important for getting a train moving and climbing gradients. Isolating a motor on a Bo-Bo would reduce the starting TE by 25%. Limiting power generally only has an effect at higher speeds.

 

[hwl]

Limiting power the advantage that the loco still has its maximum tractive effort at low speeds, which is important for getting a train moving and climbing gradients. Isolating a motor on a Bo-Bo would reduce the starting TE by 25%. Limiting power generally only has an effect at higher speeds.

Exactly!

 

[edwin_m]

Modern EMUs tend to be power-limited on the third rail so as not to overload the supply, which explains why dual-voltage units are more sluggish south of the river.

 

[contrex]

Yes and bear in mind the "pedestrian speed" routing out of Waterloo, was always an interim solution until HS1.

They used to do the ton in Kent, didn't they? In places on the SEML?

 

[Sun Chariot]

They used to do the ton in Kent, didn't they? In places on the SEML?

The three times I used that route, my services didn't get anywhere near 100...

 

[Richard Scott]

The three times I used that route, my services didn't get anywhere near 100...

Had 103mph out of a 73 and 3-REP combination many years ago, obviously not in Kent, but between Southampton and Bournemouth.

 

[Sun Chariot]

Had 103mph out of a 73 and 3-REP combination many years ago, obviously not in Kent, but between Southampton and Bournemouth.

Yes, the SW Main line enabled spirited running along several sections; especially during and since the mid-1980s upgrade in readiness for the full introduction of 442s.

 

[Recessio]

Current demand upon 3rd rail, for "day to day" train operating speeds in excess of 100mph, would risk the reliability of the current infrastructure - so, yes, we won't see that happen any time soon.
However, a class 442 did reach 109mph: on 11th April 1988, as part of the fleet proving activity.

Yes, the SW Main line enabled spirited running along several sections; especially during and since the mid-1980s upgrade in readiness for the full introduction of 442s.

I believe the 109mph reached by a 442 during trials is still the speed record for a third-rail DC system. (There's other stuff online about some units reaching 117mph, but I can't find concrete evidence).

Compare this to the fastest OHLE trains, where a TGV once reached 350mph and I'm sure there's probably some maglevs that have gone even faster...

 

[Nicholas Lewis]

Many thanks, everyone - post now corrected!

For those curious, the rectifier from a class 81 (I missremembered it as 87, whoops!) looks like this:

View attachment 180881
[credit]

I don't believe they used glass bulb rectifiers in locomotives as the mercury would have been sloshing about and hard to maintain a continuous arc let alone the risks from damage and the consequences of mercury leaking everywhere. This is also a multi phase anode version they were single phase in the locomotives. The GEC built locos used the Com Pak single phase steel units that were designed for the rail industry environment but as to whether they were prepared to sell them to competitors at the time I don't know. The 81's were BTH electrical equipment who certainly manufactured glass bulb rectifiers as they were the supplier for the track side substations for Kent Coast phase 1.

 

[siacelestar]

which is actually why we haven't seen any meaningful conversion

In Hong Kong metro system we have both 25kv AC and 1500V DC system. There were talks for conversion to AC when the two companies merged but it didn't happen.

One of the reason I heard was that they couldn't work out where to put the neutral section because the stations are too close together, let alone the cost of the conversion...

 

[hwl]

Many thanks, everyone - post now corrected!

For those curious, the rectifier from a class 81 (I missremembered it as 87, whoops!) looks like this:

View attachment 180881
[credit]

I don't believe they used glass bulb rectifiers in locomotives as the mercury would have been sloshing about and hard to maintain a continuous arc let alone the risks from damage and the consequences of mercury leaking everywhere. This is also a multi phase anode version they were single phase in the locomotives. The GEC built locos used the Com Pak single phase steel units that were designed for the rail industry environment but as to whether they were prepared to sell them to competitors at the time I don't know. The 81's were BTH electrical equipment who certainly manufactured glass bulb rectifiers as they were the supplier for the track side substations for Kent Coast phase 1.

That isn't "the rectifier" from the 81. The caption figure 99r in Gary's book has that as "Glass bulb mercury arc rectifier, in use at Laxey, Isle of Man" the photo to the left of that one figure 99l (which looks identical to the item below).

The 81s had 3 rectifiers with 6 anodes each and steel tanks, photo below of 1 of the 3 rectifiers from the AEI brochure: