Yes, there have been improvements in batteries, but they still are niche products at the utility scale. Useful for load smoothing and such at a local scale, but not at a power plant scale as is needed for the desert solar stuff.

Then there's working life and maintenance, as well as response time to changes in charge/discharge. The latter is perhaps not as important in a utility-scale system as it is in a Wayside Energy Storage (WES) system for transit use.

I was surprised but, after reading the study, intrigued by LA Metro's choice of flywheels for their WES pilot - due to maintenance needs of batteries and failure modes of batteries and capacitors causing more potential problems than mag-suspension flywheels. So "mechanical" storage might not be so bad an idea. Though if the tracks for this scheme are built to common low-speed RR standards and the equipment is just heavily ballasted standard electric locomotives I'm not sure about the maintenance issue.