When it comes to the environment the most efficient use is to leave it in the ground.

Hybrids work for trains because they are so large and don't need big swings of acceleration or to climb steep grades. They can run the diesel generators at maximum efficiency.

Battery power would be better, because you can build even larger power plants running at higher heats and not have to haul them with you, but the costs of sufficient battery is too large, so far. That is changing.