Given the mass of the cars they're talking about, they'd need substantial engineering to get a good solid level grade that's not going to subside or slip, which

Given the mass of the cars they're talking about, they'd need substantial engineering to get a good solid level grade that's not going to subside or slip, which isn't always going to be straightforward. 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.