At night, when demand is lowest it's probably actually helpful to be pulling a few MW out of the grid to avoid wind turbines having to be switched off.Again... power that could be used elsewhere in place of fossil fuel derived power, if the WCML wasn't using it. No power is really zero carbon until all of our power is zero carbon.
And it is zero carbon if that's the source of the power Network Rail are buying. To use an analogy, I don't accuse my wife of pretending to be vegetarian because I eat meat. "No one's vegetarian until everyone's vegetarian so you might as well have a bacon sarnie love" would be absurd.