Density doesn't matter for storing power by lifting up weight, what matters is total weight lifted, and how far.
As far as I can tell, the problem with the heavy rail car method is that it costs too much for the amount of storage you get. One reason I say that is that if the economics if it were good (and I assume they are pretty easy to calculate), then articles that talk about different methods for storing all that renewable energy we are getting would include it, but they rarely if ever do.
Also, unlike other methods like lithium-ion batteries and flow batteries, the various technologies involved in heavy rail cars are already well-developed, so there is little room for improvement once the first few models are developed. I mean, it is not like anyone is likely to develop cheaper ways of making rails or railroad cars.
As far as I can tell, the problem with the heavy rail car method is that it costs too much for the amount of storage you get. One reason I say that is that if the economics if it were good (and I assume they are pretty easy to calculate), then articles that talk about different methods for storing all that renewable energy we are getting would include it, but they rarely if ever do.
Also, unlike other methods like lithium-ion batteries and flow batteries, the various technologies involved in heavy rail cars are already well-developed, so there is little room for improvement once the first few models are developed. I mean, it is not like anyone is likely to develop cheaper ways of making rails or railroad cars.