Density helps. But environmental issues help more. There are only so many viable locations for large-scale hydro storage, and it always means drowning some valley. Just building a set of rails up a hill is much less environmentally invasive.<p>One of the biggest wins of this model, really, is the low environmental impact, relative to hydro or batteries. Fewer regulatory hurdles, more potential locations. But in the end, cost effectiveness is what will win, and to some degree, that will be location-dependent. Is there geography we can exploit? Will it be in a populated area? What&#x27;s the proximity to wind or solar farms?<p>Solving the storage problem in a cost-effective way is the key to transitioning from poison fuels to clean power. Once it&#x27;s cheaper than coal&#x2F;LNG&#x2F;nuclear, the world will switch over quickly.