Holy crap this childish mis-information is just bizarre. A little bit of NON-Jr. High school brilliant ***** child imaginarium info: "because there are already likely millions of them across the planet" Likely? Likely?! No not likely at all, you are really just pulling brain damaged guesswork out of your fifth point of contact right from the start. No, just no, there are huge numbers of mileage of old shafting, damn little of it is anything like the near vertical/vertical bore that is needed to store and dispense the kinetic energy under discussion even if rails were re-installed and maintained in steeply sloped shafting, at a huge labor cost for poor energy storage results. "relatively cheaply converted for this purpose" Yeah, nope NOPE NOPE, the cost to rework shafting to store a significant amount of kinetic energy in any civilized country and operate and maintain it and lease it and put in the needed infrastructure for miniscule energy storage in just a few or one shaft with effective safety organizations that will work against you murdering large numbers of disposable mining workers ala Qatar olympic construction mass murders for your idea is quite large, just to convert the very damn few miles of vertical shafting in an abandoned and who knows how badly deteriorated un-maintained hole, and this does not begin to take into account the construction etc. costs of installing and using the generators to convert that kinetic energy to electricity. How deep is your average mine? Less than 3/4 of a mile... how many vertical or near vertice meters of shafting of all sorts from ventilation to whatever? How many steep or vertical; shafts on any one site 4? 8? 2?. Those connected to the power grid abandoned holes? "and are already connected to the power grid." Not really very connected to any grid whatsoever, most dead mines and abandoned shafts are where they are, you have a bare few hundred MAYBE in places like Germany where useful power transmission infrastructure exists and the vast majority in forgotten wastelands far from any major power consuming/handling areas, have you even looked at the research showing why powering the planet with solar power plants in the north African deserts and other wastelands is not feasible? POWER TRANSMISSION IS A HUGE LIMITING FACTOR! Yeah all those abandoned mine shafts... mostly filled with water far beyond any energy storage value capacity to pump out and convert to gravity storage in places like Eastern and Northern North America. ETC. ETC. ETC. Childish dreams and fevered pre-teen speculations on how an imaginary world might work are not very appropriate for presentation in the manner of this fairytale, put this sort of thing out as the early days wildeyed spitballing it is, not as something somehow to be viewed as reasonable or sane or otherwise taken seriously as it currently stands.