Forrest pursues gravity-charged electric train to shift his iron ore

There's already such a train in Sweden. It's 1920s technology :)

BTW you don't need big batteries. You run 2 trains and make the one going down power the one going up (without ore). And the surplus goes to the power grid. Batteries (other than a small buffer) would just be wasted weight and money (and would introduce consumables where no consumables are needed).

Other option is using the grid and profiting from daily energy price differences. I'd assume in Australia energy is significantly cheaper when sun is shining because of solar panels. So run the trains up during the day and down during the night. Congratulations - you made your trains into a grid-scale batteries. You can profit and solve the biggest problem with renewables at the same time.

Putting enough batteries to let every train go up by itself is such a waste.