Don't know if this has already been discussed but I have doubts about OHL being better than 3rd rail. Often see delays due to OHL faults reported but very rarely power supply problems on 3rd rail lines. OHL is vunerable to high winds and not winter proof. In certain conditions enough weight of ice can build up to bring the wires down requiring 'icebreaker' trains to run overnight, if they're not needed the first train has a tough time and has to be double unit. Heating strip on 3rd rail has proved a cheap and effective measure against icing.
On much of the old Southern Region where speeds are low and stations close together 3rd rail has distinct advantages, heavy transformers for high acceleration from starting placed lineside makes the large number of trains cheaper, lighter and more reliable. I wonder what regular travellers will make of the new improved system (after all the disruption installing it) when their trains are noticably slower to get going and grind to a halt at all times of year!
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I think someone needs to look into the relative merits better, you can deliver significantly more power, significantly more efficiently by using OHLE...
You'll also find that the "Oh, it's already DC" argument falls very much on it's backside too, every new train since the 90s uses AC traction motors, granted from a DC Bus Bar but the increased draw capabilities of AC over DC very much make up for the extra weight, and allow the use of standardised components, changing 750V DC to something that isn't 750V DC to use is very expensive component wise, and non-standard.
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Stupid question probably, but can you have a AC 3rd rail?
To achieve the same RMS power throughput you'd need a 1.414 times higher peak voltage and current, meaning you'd still need a significant amount of feeder stations to run at a low enough voltage to prevent flash over.
The insulators would have to be more complex too as there tends to be communications equipment around the same kind of voltage that wouldn't be immunised against high current low voltage AC in an environment where transinductive conduction is very easy. This isn't so much of an issue with insulated ground 25kV cables as the current is about 80 times lower.