From site: "3GW Regional Energy Storage Hubs. Energy capacity may range from 4 to 16 hours duration at full power output."

The 3GW is big but the duration is small, so I'd guess they're doing short term grid stabilization, competing more with flywheels than hydro. That's just a hunch, but if that were true, it'd explain how you could get something really effective without a lake-sized train. (Some videos online suggest they move a bunch of heavy cars on the same track independently, so they probably also get much more weight than a normal train.)

My limited understanding of pumped hydro specifically was that it really was basically just a turbine operating in two directions, crazy as that sounds. Even so though, I can easily imagine big cost savings just from not having to engineer around water. That stuff doesn't compress, you design or run these wrong and they'll just mercilessly explode: https://www.youtube.com/watch?v=6hVUeNp3o3M

Being able to build anywhere is really neat too.

So, grid storage tends to fracture into a half dozen specific types of storage, all based on duration, spin up time, geography, fuel, etc. There are like 18 different tradeoffs, and you can specialize in any one of them. Pretty much any new grid storage technology is going to add to the field rather than displacing anything, because the grid needs arbitrage in basically all 18 different dimensions.

Thanks for the link. There are some cool ARES videos on youtube too.