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Comment by ginko | Hacker News Reader

I guess it'd be a big safety headache but perhaps the train could be powered through the rails? To avoid accidents the rails could be split into sections that only get powered when a train is directly above them.

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The rails are already used for ground/return current. The single overhead wire is at 25kV (or higher for this speed test).

It is only where the vehicle has rubber tyres (i.e. trolley buses) where there are two overhead cables.

Changing the current system would be prohibitively expensive. I also imagine that you have pretty bad losses at 25kV AC if your conductors are touching the ground and are potentially wet or covered by snow. It would also be a fire hazard I guess.

Plenty of metro rail systems use a third rail today, even outside. Maybe their losses are more manageable because their systems are relatively short compared to long distance rail, but it doesn't seem like an insurmountable obstacle.

Metro rail runs at a much lower voltage afaik.

They do this in South East UK using a dedicated third rail for power. As I understand it, the limit is 100mph for this technology, although I don't know the physics, so it's not even fast enough for classic High Speed

Part of the problem is the low voltage conventional third rail systems use. In the UK, third rail voltage is 750V, compared to 25kV for overhead lines, so third rail currents get pretty high. You already get get pretty good arcs when the contact shoe moves on and off the third rail going through junctions. And you can't really increase the voltage very much before the rail starts to arc to ground, especially when it's raining and the insulators get wet.

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