A dude on the Danish engineering society worked through the math, and indeed, rail cars don't seem feasible. Just too expensive.
I don't have too high hopes for the sphere idea either - operating turbines in seawater 700 m down seems too complicated compared to transporting the energy to land and storing it there, somehow.
I don't have too high hopes for the sphere idea either - operating turbines in seawater 700 m down seems too complicated compared to transporting the energy to land and storing it there, somehow.