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Wouldn't it be easier to have DC lines running along side the tracks and have some sort of roller contacts (if the lines are flat and the rollers are spring loaded) or brushes connect to those DC lines instead of overhanging lines (AC or DC) for electric trains? Everything from maintenance and capital cost would be reduced. Yes, having exposed high-voltage electrical lines close to the ground is risky and weather might also be a problem but these seem like problems much easier to solve than having a massive infrastructure of overhanging cables, pantographs, maintaining tension, aerodynamic inefficiencies with high speeds, etc.

Am I missing something here? Please let me know if I am wrong.

JRE
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Doctor Pinocchio
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    As you say, there are problems with each. People picked a route and went with it. In some cases they went one way, in others they went with another. Add road maintenance to your list. Easier is opinionated. – Abel Jun 19 '22 at 11:09
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    https://en.wikipedia.org/wiki/Third_rail for examples using this system – bobflux Jun 19 '22 at 11:24
  • If you ever use a subway the power is usually provided from the ground like you suggest since there is little chance of people/animals touching subway rails and getting electric shocks underground. – user1850479 Jun 19 '22 at 12:43
  • What's so special about DC that you suggest *both* moving to a ground rail *and* using DC and not AC? Why conflate the two? – Kuba hasn't forgotten Monica Jun 19 '22 at 18:18

3 Answers3

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In fact, there are a lot of train systems with a power line close to the ground. E.g. most (all?) subway systems.

What all these have in common is that the access of misguided humans, animals, vegetation, water and machinery is heavily restricted.

If such a restriction is not possible, few meters distance from the ground are a good start.

Edit:

Looks like AC and DC underground train systems exist in comparable numbers.

As per the comment from @Kevin White, the first ever electricity-powered underground train system (the London Underground) is DC and other DC-powered systems exist. @GrapefruitIsAwesome comment mentions PATCO speedline.

My native Sofia metropoliten is AC 875V, so are all other that are technologigally derived from the Moscow "Metro".

fraxinus
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  • [PATCO](https://en.wikipedia.org/wiki/PATCO_Speedline) "All PATCO trains are electrically powered. Power comes from a top contact covered third rail at 750 V DC." – GrapefruitIsAwesome Jun 19 '22 at 11:57
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    The oldest underground system in the world, the London Underground, has used DC since it was electrified about 1890. It uses a 4 rail system. https://en.wikipedia.org/wiki/London_Underground_infrastructure – Kevin White Jun 19 '22 at 16:36
  • @KevinWhite this is maybe exactly because it is the oldest. AC became a thing somewhat later. – fraxinus Jun 19 '22 at 16:56
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There are some trains that use DC instead of AC (mostly in city areas).

The reason that the power lines are often hanging is that air is a very good, reliable and cheap isolator. So because of isolation, it is much easier, cheaper and safer to have the high voltage lines hanging.

Stefan Wyss
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  • Nobody would use soil to isolate power DC lines. Of course a buried power line has an isolation of high quality plastic. – Uwe Jun 20 '22 at 07:40
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The AC power from the overhead line is transformed down to a lower voltage and than rectifed and supplied to the inverter for the motors.

In the DC trains the transformer and rectifier can be landside, and therefor don't need to be accelerated and decelerated, saving on energy and (usually restriced) space in the train.

The advantage for AC trains is the usually much higher voltage and therefor lower energy losses in the line.

Often you see AC in long distance trains, while urban systems use DC.

Christian
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