Ballast is > 100x cheaper than powerwalls by the kWh in your equation[1]. 10 tons of rock just 5 m^3, you can put hundreds or thousands of this size of rock in a train[2]. You load/unload ballast instead of having all of it statically bolted onto rails and cars, so you get by with 1/50..1/100 carrying capacity[3] per trip vs total ballast amount.

The amount of raw materials (lithium mining and refining etc) required by the powerwalls is probably much higher too - good lithium deposits have ~ 1/100 of lithium content and turning ore into batteries is a heavy process consuming a lot of water, electricity, fossil fuels (ore transport/mining) etc. Not to mention limited battery service life.

[1] 2x powerwalls = $16000, even building material grade gravel is < $300 per 10 tons (50x cheaper) but here you could use sand or something from the site.

[2] big ore transport trains carry 10000-40000 ton loads, so 1000-4000 of our 10t rocks per train. You would probably use a smaller train for energy storage purpouse since the distance is small and you can have quick load/unload turnarounds

[3] if aiming for 1-day charge capacity, you can fit 100 15-min round-trips in 24 hours, or 50 30-min round trips