Pete.
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- 14,148
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- Kent, UK
The conservation and conversion of energy - kind of a holy grail. Friction - from bearings and wind is an issue which is there whether the conversion is taking place or not. I haven't heard if its been developed to something useful. Compressing air is another one (mentioned allready).
Underground stations are set at the top of a slope nowadays so the train rolls out of the station the slope helps to accelerate it, then slow it down when it approaches the next station.