It can be depressing to reject so many ideas for power generation or bulk storage of energy. That makes it refreshing to read today's news about a new method that IMO sounds 100% credible in their claims, and that has a potential scalability that could far exceed pumped hydro. http://fortune.com/2016/05/22/energy-storing-train-nevada/ The main reason that I find this easy to believe is that electric locomotives with regenerative braking systems have been in use for many years. No new technology needs to be researched or developed. The locomotive designs have already been refined over the years. Even their efficiency and cost claims can be backed by the records of extensive real-life experience, not just calculations or laboratory experiments. The number of suitable locations for this scheme are far more than suitable pumped hydro locations. That's why I think they could really scale it big. If I ever learn the names of the engineers who first proposed this, I would like to nominate them for awards.