Based on the very lightweight comparison with pumped hydro storage, I think they realise it's going to be hard to compete with. How are the costs lower than a pipe a pump a pond and a turbine?
I don't have hard data - but wouldn't engineering a rail system not be as hard, and need less civil engineering work?
As you say, however, it's very geo/topo specific - so in places like North Wales, pumped hydro is still clearly the way to go, but in the Mojave, this rail based system could work well.
It reminds me of the defunct system we had in a house I used to live in - 8" or so diameter shaft in the basement floor with a weight, a cable, a winding, and a dynamo - you'd have the servants crank it up during the day so you could have electric light at night. Quite popular in the late nineteenth century among early adopters of electricity.
In general there is no silver bullet for the electricity system, rather, a better metaphor is a stack. This tech might fit one space in the stack.