Is third rail bad at acceleration?

[Deepgreen]

The 700s have an unused capability for far greater acceleration but it requires major traction supply improvements to use it. I suspect it will never see use.

 

[43066]

Re. The exact thread title, I have no idea. I have never seen a third rail accelerate!

Networkers on the other hand (especially in METCAM guise) are pretty rapid, and about the quickest third rail units.

 

[AM9]

The 700s have an unused capability for far greater acceleration but it requires major traction supply improvements to use it. I suspect it will never see use.

The problem is that would only be really useful on stretches of the BML, and with the volume of other traffic on the line it would just mean that they close up on the train in front and run on yellows.

 

[TheGrew]

TBH the place where I think replacing 3rd rail with OHEL makes the most sense on the UK network is what the Electric Spine proposed, Basingstoke - Southampton as combined with other elements a great deal more freight could be electrically hauled.
If I were going to go about replacing 3rd rail with OHEL I would probably start furthest away from London and work inwards as once you get closer to the capital you are more constrained by traffic and signal distances.

 

[Deepgreen]

Yes, third rail is terrible at acceleration - because it's fixed to the ground.

B-boom tish.

(Sorry - couldn't help myself).

Actually OHLE is fixed in more positions than third rail, which only rests on the insulators and is only fixed at cable joints.

== Doublepost prevention - post automatically merged: 20 Dec 2023 ==

The problem is that would only be really useful on stretches of the BML, and with the volume of other traffic on the line it would just mean that they close up on the train in front and run on yellows.

Indeed - I believe it was intended for the core where rapid throughput would be more useful.

 

[Nicholas Lewis]

The 700s have an unused capability for far greater acceleration but it requires major traction supply improvements to use it. I suspect it will never see use.

The 8cars go far better than a 12 as the current limit is per train so they get a bit more oomph.

 

[AM9]

Actually OHLE is fixed in more positions than third rail, which only rests on the insulators and is only fixed at cable joints.

== Doublepost prevention - post automatically merged: 20 Dec 2023 ==


Indeed - I believe it was intended for the core where rapid throughput would be more useful.

So to summarise the impact of the limitations of 3rd rail in the south:

within the metro area, (up from Wimbledon, East Croydon, Orpington/Sevenoaks and Dartford), services run much as they would under OLE, most of the 3rd rail issues have been dealt with by massive investment in a massive number of feeds, - in some cases at a per block level​

outside that on the main lines, apart from the route of the ex-CTRL supply reinforcement, 3rd rail really does impact on the allowed performance of a capable and relatively modern group of fleets. Probably the biggest loss is the SW mainline, where with a good alignment, 100mph (or even 110mph in places) should be attainable by all EMUs west of Wimbledon to Eastleigh, irrespective of their length.​

The above is in addition to the overall shortfalls of 3rd rail, i.e. a lacklustre performance in icy weather, some safety compromises and a relatively poor power supply efficiency.

 

[edwin_m]

TBH the place where I think replacing 3rd rail with OHEL makes the most sense on the UK network is what the Electric Spine proposed, Basingstoke - Southampton as combined with other elements a great deal more freight could be electrically hauled.
If I were going to go about replacing 3rd rail with OHEL I would probably start furthest away from London and work inwards as once you get closer to the capital you are more constrained by traffic and signal distances.

And the energy saving reduces because the distance between trains and substations or other trains regenerating is that much less.

There was a RSSB study in 2010 that proposed just that approach. It was also influenced by most of the units on the outer part of what is now SWR being easily capable of adaptation to dual voltage, unlike further in where the 455s were still on DC camshaft control. The Networkers on SouthEastern would also have been difficult to convert.

 

[Recessio]

I know in some tram systems and other countries there are trains that use DC OHLE. But has there ever been AC third-rail systems? Would there be any benefit over a DC system (especially in terms of acceleration) , or is it a terrible idea?

 

[londonmidland]

How does a DEMU such as a Voyager/Meridian compare with acceleration? I’ve always found them quite rapid.

 

[edwin_m]

I know in some tram systems and other countries there are trains that use DC OHLE. But has there ever been AC third-rail systems? Would there be any benefit over a DC system (especially in terms of acceleration) , or is it a terrible idea?

It's a terrible idea* due to something called the skin effect, which means that an AC current only flows near the surface of a conductor. At 50Hz the depth is a few millimetres, which not by coincidence is about the radius of a 25kV conductor. If AC was fed through a third rail then the bulk of the cross-section wouldn't be conducting any current, so the resistance and the losses would be much worse than on the equivalent DC system.

*but no doubt someone somewhere has done it.

 

[Nicholas Lewis]

So to summarise the impact of the limitations of 3rd rail in the south:

within the metro area, (up from Wimbledon, East Croydon, Orpington/Sevenoaks and Dartford), services run much as they would under OLE, most of the 3rd rail issues have been dealt with by massive investment in a massive number of feeds, - in some cases at a per block level​

outside that on the main lines, apart from the route of the ex-CTRL supply reinforcement, 3rd rail really does impact on the allowed performance of a capable and relatively modern group of fleets. Probably the biggest loss is the SW mainline, where with a good alignment, 100mph (or even 110mph in places) should be attainable by all EMUs west of Wimbledon to Eastleigh, irrespective of their length.​

The above is in addition to the overall shortfalls of 3rd rail, i.e. a lacklustre performance in icy weather, some safety compromises and a relatively poor power supply efficiency.

There are many more lines that have been uprated to high current railway standards. The whole of the Brighton Main Line, both old boat train routes to Folkestone and SW to Southampton. Other routes have had localised reinforcement to allow longer trains/higher frequency. Its fair to say icing is top contact Achilles heel but incidents are infrequent and its certainly more resilient than OLE to mechanical damage. SW has the power supply now but no interest from management to get the units remapped as happened with the electrostars.

 

[TheGrew]

Yeah I remember them doing the upgrade to the SWML when the Desiros were new. I think part of the quoted reason was increased power draw due to AC though traction would have certainly contributed.

 

[Bikeman78]

I know in some tram systems and other countries there are trains that use DC OHLE.

The Dutch railways uses 1500V DC. The 1970s sprinters accelerated from 0-70 mph in 40 seconds flat, as did the class 1700 locos on the short double decker sets. The reason I mention 0-70 rather than 0-60 is because I got distracted the first few times and when I glanced back at the speedo it was already well past 60! This was not a one off performance. It happened daily hundreds of times.

EDIT: see video below. Fast forward to 9:40 where the train departs flat out from Amsterdam Sloterdijk. For context, these units were built the same time as the 313s.

 

[Recessio]

It's a terrible idea* due to something called the skin effect, which means that an AC current only flows near the surface of a conductor. At 50Hz the depth is a few millimetres, which not by coincidence is about the radius of a 25kV conductor. If AC was fed through a third rail then the bulk of the cross-section wouldn't be conducting any current, so the resistance and the losses would be much worse than on the equivalent DC system.

*but no doubt someone somewhere has done it.

Ah I see, I figured there was probably some reason why. Thanks!

 

[TheGrew]

The Dutch railways uses 1500V DC. The 1970s sprinters accelerated from 0-70 mph in 40 seconds flat, as did the class 1700 locos on the short double decker sets. The reason I mention 0-70 rather than 0-60 is because I got distracted the first few times and when I glanced back at the speedo it was already well past 60! This was not a one off performance. It happened daily hundreds of times.

EDIT: see video below. Fast forward to 9:40 where the train departs flat out from Amsterdam Sloterdijk. For context, these units were built the same time as the 313s.

The Dutch are very much considering upgrading their system to 3kv DC as per France and Belgium. Dutch trains are reasonable accelerators but have quite low top speeds compared to similar units in other countries. This isn't so much of an issue where distances are comparatively short though.

 

[zwk500]

Probably the biggest loss is the SW mainline, where with a good alignment, 100mph (or even 110mph in places) should be attainable by all EMUs west of Wimbledon to Eastleigh, irrespective of their length.

Slight point of pedantry - 100mph on the SWML doesn't start until Byfleet. There's a previous thread with the mileages, but 110mph running on all existing 100mph section gains you very little time. The advantages of conversion would be efficiency and safety.

The Dutch are very much considering upgrading their system to 3kv DC as per France and Belgium. Dutch trains are reasonable accelerators but have quite low top speeds compared to similar units in other countries. This isn't so much of an issue where distances are comparatively short though.

My understanding was that there was a long-term aim but no serious programme to convert to 3KV.

 

[edwin_m]

Neighbouring Belgium uses 3kV but has adopted 25kV for recent work.

 

[Bikeman78]

The Dutch are very much considering upgrading their system to 3kv DC as per France and Belgium. Dutch trains are reasonable accelerators but have quite low top speeds compared to similar units in other countries. This isn't so much of an issue where distances are comparatively short though.

As a child in the 1980s I was always fascinated by the modern looking yellow trains over there. Even the 1964 units took off like a rocket but the SGM Sprinters were off the scale compared with anything else I encountered.

== Doublepost prevention - post automatically merged: 21 Dec 2023 ==

Neighbouring Belgium uses 3kV but has adopted 25kV for recent work.

Only in the south, and for the high speed lines. The new wires around Mol are 3kV.

 

[AM9]

Slight point of pedantry - 100mph on the SWML doesn't start until Byfleet. There's a previous thread with the mileages, but 110mph running on all existing 100mph section gains you very little time. The advantages of conversion would be efficiency and safety.

Slightly better on the stoppers' or intermediates' acceleration helping the paths issue, and a lot better on freight.

 

[zwk500]

Slightly better on the stoppers' or intermediates' acceleration helping the paths issue, and a lot better on freight.

If freight can use the wires, which isn't many flow routes at the moment and even fewer terminals. Class 93 and 99 could change that though.

== Doublepost prevention - post automatically merged: 21 Dec 2023 ==

Neighbouring Belgium uses 3kV but has adopted 25kV for recent work.

It's only adopted 25KV where the line already has an end-on connection to French 25KV AC lines. EDIT: And only on a handful of lines.

 

[43096]

It's only adopted 25KV where the line already has an end-on connection to French 25KV AC lines.

Or Luxembourg.

 

[stuu]

It's only adopted 25KV where the line already has an end-on connection to French 25KV AC lines.

Actually only Luxembourg, apart from LGV Nord

 

[zwk500]

Actually only Luxembourg, apart from LGV Nord

Good point, correction noted! (They do have a connection to a french 25kv ligne classique at the BE/Lux/FR tripoint, but Brussels-Luxembourg is the corridor they're converting slowly).

 

[edwin_m]

Actually only Luxembourg, apart from LGV Nord

I think I can see a 25kV route into France near Longwy that misses Luxembourg by a few hundred metres - boundary captions seem to have gone a bit haywire on openrailwaymap but I've cross-checked with Google.

 

[notadriver]

How does a DEMU such as a Voyager/Meridian compare with acceleration? I’ve always found them quite rapid.

It’s about 60 seconds to 60 mph so in between an AC emu and DC third rail EMU.

 

[stuu]

I think I can see a 25kV route into France near Longwy that misses Luxembourg by a few hundred metres - boundary captions seem to have gone a bit haywire on openrailwaymap but I've cross-checked with Google.

Fair enough, that will teach me for not zooming in enough! I would be surprised if much traffic uses that link though

 

[Richard Scott]

The Dutch are very much considering upgrading their system to 3kv DC as per France and Belgium. Dutch trains are reasonable accelerators but have quite low top speeds compared to similar units in other countries. This isn't so much of an issue where distances are comparatively short though.

No 3kV in France as far as I'm aware.
Don't think and 1.5kV has been converted to 25kV either. There are many places, though, where additional electrification has been at 25kV tagged onto existing 1.5kV network.

 

[TheGrew]

No 3kV in France as far as I'm aware.
Don't think and 1.5kV has been converted to 25kV either. There are many places, though, where additional electrification has been at 25kV tagged onto existing 1.5kV network.

You are correct, my mistake.

 

[zwk500]

Don't think and 1.5kV has been converted to 25kV either. There are many places, though, where additional electrification has been at 25kV tagged onto existing 1.5kV network.

The Dutch looked at it several decades ago, AIUI, but decoded conversion to 3KV was better value given the Belgian network to the south and the fact the Germans are on 15KV 16.67hz.