Also molten salt:

https://www.solarthermalworld.org/news/molten-salt-storage-3...

People talk about making "personal" molten salt units for the home, which seems a little terrifying.

I wonder what the local zoning commission has to say about the guy building a personal toxic metal death ray in his backyard?

That is why I will never live in a HOA neighborhood.

The zoning commission runs at the town/country level, so even if you don't have HOA busybodies you still need to get the permits. The law is usually worded such that if you're doing something abnormal you don't get a "there is no rule that says a dog can't play baseball" loophole, you instead have to bring it up before the county council and justify the project.

Seems the train idea is similar to the old counterbalance system for trollies in Seattle.

Hydro pump storage works because you don't have to pay for the weights. Building a train is worse than batteries in terms of economics.