https://www.greentechmedia.com/articles/read/First-Grid-Scal...
We have a pumped hydro solution where I live. It's huge and only supplements a small power plant.
"One of the striking things about the ARES project is its size: 50 MW of power capacity and 12.5 MWh of energy. That would be large for a battery storage project, but for ARES, it is on the low side. “Fifty megawatts doesn’t get us to economies of scale,” CEO James Kelly said. “We are more efficient as we get larger.”
Kelly said ARES projects can be sized anywhere from 50 MW to 1 GW. “If we had a 500 MW project, we could double the capacity and it would only increase capital costs by 20%,” he said."
Could they perhaps achieve high enough mass in a small volume by using eg. lead?
Could it be that a train is more efficient at converting potential energy than a water turbine?
Could it be that they have a longer drop?
You gotta show your assumptions if you're arguing through back of the envelope calculation.
Not all locations have a lake (or 2) conveniently located near a large elevation drop.
The article says "They move up and down an 8-degree slope with an elevation change of about 3,000 feet", that's a lot of energy.
A reservoir 10m deep and 45m square at the same elevation has the same potential energy. If you can find a convenient 1000m plateau I think it would probably be cheaper to build a reservoir on top than to build the tracks and trains.
Now I wonder how fast these railcars move.
I wonder how hard it is to scale though.
See this for a comprehensive annual survey by Lazard on storage technologies and costs
http://blog.rmi.org/blog_2016_01_21_how_much_does_storage_re...
You could build surface level reservoirs as two tank hydroelectric dams, and then use the water as an energy reserve as well as freshwater reserve.
Pumped hydro is probably the best bet, but is very geography dependent. Adding transmission is a non-trivial investment.