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

[Taunton]

This one has rumbled on here for a long time, including by some of the contributors above! One of the issues is proponents of either system extolling all the virtues they can come up with, while staying silent about any downsides.

3rd rail is a sight easier, quicker and cheaper to install. Having watched it actually go in one weekend I'll offer by a factor of 10 times. It doesn't suit high speed running (unless you are doing Waterloo to Bournemouth it seems), but does suit Metro systems, which worldwide seem to use little else, even new ones - why does the Dubai Metro, for example, brand new and a substantial system, not use OHLE?

End of the day it's horses for courses, which is why saying Reading to Gatwick, with three existing third rail sections interspersed with two non-electrified bits, should have the latter done with OHLE, just seems silly. About as silly as a little 3rd rail bit over the Border Bridge!

 

[WirralLine]

Composite conductor rail, mostly aluminium for lower resistance with a steel layer on top to reduce wear from the shoes. This presumably allowed fewer substations while keeping voltage drop within limits. I don't think it was used anywhere else so I assume there were problems with it.

Pretty sure there's a lot of this rail in use on the Merseyrail network. It's much thinner, and is a light silver/grey colour on the bottom 3/4 of it. The majority of the Hooton-Chester/Ellesmere Port extension of 3rd rail in the 90s uses it, which I believe was done on the cheap although it can be found in places all over the network.

The Chester end has suffered from low power since day 1 and has had "E" restriction boards up for god knows how many years. Pretty sure it was being discussed in another thread recently that the whole south section of the Wirral Lines are fed from Bromborough grid, with a substation at Hooton and Capenhurst only.

 

[edwin_m]

3rd rail ... doesn't suit high speed running (unless you are doing Waterloo to Bournemouth it seems), but does suit Metro systems, which worldwide seem to use little else, even new ones - why does the Dubai Metro, for example, brand new and a substantial system, not use OHLE?

100mph is the maximum on the Bournemouth line, which isn't really high speed these days.

As mentioned above, third rail for a metro can significantly reduce the cross-section of the tunnel, which has a big impact on costs, and a high frequency of trains making numerous stops suits a system where the transformer is in the substation not on the train. Another factor is that metros don't generally have level crossings and maintenance usually involves isolating the power during non-traffic periods, so there is less electrical hazard than on a third rail main line railway.

 

[JamesT]

This one has rumbled on here for a long time, including by some of the contributors above! One of the issues is proponents of either system extolling all the virtues they can come up with, while staying silent about any downsides.

3rd rail is a sight easier, quicker and cheaper to install. Having watched it actually go in one weekend I'll offer by a factor of 10 times. It doesn't suit high speed running (unless you are doing Waterloo to Bournemouth it seems), but does suit Metro systems, which worldwide seem to use little else, even new ones - why does the Dubai Metro, for example, brand new and a substantial system, not use OHLE?

End of the day it's horses for courses, which is why saying Reading to Gatwick, with three existing third rail sections interspersed with two non-electrified bits, should have the latter done with OHLE, just seems silly. About as silly as a little 3rd rail bit over the Border Bridge!

The Barcelona Metro, which has lines varying in age from 10 years old to 100 years old is all overhead conductors at 1200-1500V DC.
The Delhi Metro which the oldest bit is only 25 years old chose 25kV AC overhead, so I don't think you can generalise anything from whether a particular system chose AC or DC, it most likely depended on what the contractor building it was most familiar with.

 

[QueensCurve]

Thanks both for the technical stuff. Weren't the contact wires used on the Woodhead DC Overheads considerably thicker and heavier than what we use on the 25kv 50hz, too?

Yes. Last time I looked, many years ago, the contact wire was noticeably thicker on platforms 1-4 at Manchester Piccadilly than the other platforms.

 

[Zomboid]

The Barcelona Metro, which has lines varying in age from 10 years old to 100 years old is all overhead conductors at 1200-1500V DC.
The Delhi Metro which the oldest bit is only 25 years old chose 25kV AC overhead, so I don't think you can generalise anything from whether a particular system chose AC or DC, it most likely depended on what the contractor building it was most familiar with.

I think you probably can generalise, but there are exceptions.
Crossrail being one, though it's clear in that case that making the tunnels big enough to work with 25kV would be simpler than dual voltage trains switching over at either end - I bet it was considered at option selection though.

 

[Taunton]

Yes. Last time I looked, many years ago, the contact wire was noticeably thicker on platforms 1-4 at Manchester Piccadilly than the other platforms.

Yes, that's because for original 1,500V DC the current is much higher than 25kV. I believe it was said that when the wiring was replaced on the GEML a significant proportion of the project cost was covered from the scrap value of the old onetime DC wiring compared to the replacing AC wiring.

 

[edwin_m]

I think you probably can generalise, but there are exceptions.
Crossrail being one, though it's clear in that case that making the tunnels big enough to work with 25kV would be simpler than dual voltage trains switching over at either end - I bet it was considered at option selection though.

Tunnels bored by TBM have to be circular, and depending on the train structure gauge and evacuation walkway dimensions there may be room for OLE without making it any bigger. Also, for higher speed routes, extra free cross-section reduces pressure effects and must save some amount of power consumption. So for these types of metro an AC overhead wire may not add to the cost.

A cut and cover tunnel may be a rectangular section, in which case overhead wire may have more of a cost impact by forcing the track to be a bit lower.

 

[Energy]

Metros are different to the main line. DC allows the tunnels to be built smaller, and more frequent distribution sites can be advantageous in highly intensive service areas.

I wouldn't say it's advantageous, but the frequent feeder stations aren't as much of a concern for metros, where the distance covered is less. AC electrification is more efficient over long distances, but requires heavy transformers and rectifiers on board.

Didn't a NIMBY suggest 3rd Rail for Sydney Gardens and around Bath Spa?

Possibly, though an electrification design was agreed upon in 2015. [source]

Image Description - A 3D model of the cantilever design for electrification through Bath.

 

[Sun Chariot]

Yes. Last time I looked, many years ago, the contact wire was noticeably thicker on platforms 1-4 at Manchester Piccadilly than the other platforms.

I now need to peruse my 35mm photos taken at Manchester Piccadilly from 1989-1990! I stood there often enough but I completely failed to spot the subtle difference. Face palm!