It includes the cost of maternity, the increased propensity of taking sick days, generally shorter service length - these are all direct costs of employing women. I used to attend business change meetings in a previous life and our COO, a woman, was previously the COO at the DWP and a director at Cabinet Office and gave a compelling presentation about the costs of employing people. Women do cost more.
This isn't to say "you shouldn't employ women" or that women should be disfavoured in case they start a family. This would be flagrantly illegal and outrageous. But, train companies should be mindful about what taking on more women actually means for them.
I think the drive to recruit women is a good idea. Properly targeted, accurate and honest job adverts will find the right people. Fewer women than men really want a technical and repetitive job where you spend very long periods in isolation with nobody to talk to, but these people do exist and efforts should be made to recruit them. As you say earlier in the thread, does the current demographic makeup of the grade accurately reflect those best qualified to be there? Probably not - so let's change it.