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You would need a lot less rail cars than that. Rails cars move the weights between the low and the high point, but they are re-used multiple times in a cycle, going back and forth constantly.

It wouldn't be very efficient to have a mobile piece of tech sitting idle for 99% of the time.

The main constraint would be to make the maximum use of cheap technology that is standard today. 100t freight cars are pretty standard.

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Edit: nevermind, I was wrong

That's not how the proposal works. Each car picks up a box of rocks on a hydraulic jack and hauls it up the hill and then sets it down again and gets another one. Maybe the railcar makes 10 or 100 trips during the day.

Yes, you're both right. My fault – I misunderstood the ARES concept.

All three of you are right: Ares advertises two different concepts, only the one they call "grid storage" uses more ballast than they can fit on their rolling stock.

The one they call "ancillary services", intended only for short term grid regulation/stabilisation", uses fixed ballast that is never removed from the trains during regular operation. This is what the first prototype will be.

Everybody wins!

But you still need many cars to a supply high volume of energy per unit of time. You could do the whole project with a single train, but then it would be very slow at storing and retrieving energy.

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