Sounds like a variant of Electric Mountain in the UK. The same thing is done, only instead of moving trains up the hill they move water instead. There's more in the Wikipedia article - essentially though, this idea works fine.
Can't help thinking that water is the better option (where it's an option at all). Whether or not this method is, as claimed, more efficient - which caim, frankly, strikes me as more of an issue of politics and engineering than of physics - it's still going to take one heck of a lot of trains to store the same amount of mass (and therefore potential energy) as one decent-sized reservoir.
Elevated CO2 levels might, up to a point, have at least some useful, maybe even vaguely beneficial, effects (I'm no expert - I'll defer to those who are). But even if that's true, like everything in the whole climate discussion, it's wise not to forget that changes aren't likely to just stop at some convenient point - they're not only likely to keep going, but in a worst case to snowball utterly beyond our ability to do anything but hang on and watch (in a worst-case scenario, that may already be the case).
To draw a vague (but possibly familiar) parallel...
Anyone who's done any home brewing, or who simply understands roughly how brewing works, knows that it takes yeast. And yeast feeds on sugars. Add a little bit more sugar to your brew, you'll increase yeast productivity. Unfortunately, that's only part of the story - because the effect doesn't scale indefinitely. Add TOO much sugar, and the yeast won't grow at all. (It's also worth pointing out that things don't normally exactly end well for the yeast. which eventually dies from its own waste products - roughly what we're in danger of doing, in fact. But that's a slightly different point.)
Back them up.
I have slides that my parents took. Sadly, the colour decayed in many of them, and my mother threw far too many of the "worst" ones out (including ones of several episodes in my life that I'd love to have the record of now). A real shame, as when they came into my possession on her death, and I scanned such as were left so that various branches of the family could have copies, the scanner software was perfectly capable of restoring the colour balance. But the message is still there - slides decay, and they're irreplaceable. Take a backup.
I keep my important files under single directories on a hard drive on each of my two desktop machines. I back them up regularly with an incremental backup, excluding deletes, to (a) a second hard drive internal to the machine, and (b) an external hard drive. I don't delete images from my cameras until I'm happy that I've backed them up successfully, both internally and externally. I also swap to new directories on the backup drives from time to time and do a new, complete backup, and to new drives every couple of years or so. I check-sample the content of the originals and backups every once in a while. I can't guarantee that I won't lose something important eventually, but in practice I have quite a few copies of just about anything you care to name, and it's going to take something pretty extreme to do it.
"To IBM, 'open' means there is a modicum of interoperability among some of their equipment." -- Harv Masterson