Looking at the houses (sorry, catching up), if the railway was in a tunnel, you could fit each house with French Windows at 1st floor level with a link to the new garden, so yes, down stairs there would be a loss of light but upstairs you gain a new feature and direct access to the garden. My late parents in law lived in such a house, but arranged with the bed rooms downstairs (cooler) and the living room and kitchen upstairs with the link to the first floor garden, all due to being built into a hillside. So I'd also offer to fund internal rearrangements if wanted. I am sure this is doable in some way.
I would guess Network Rail will be keeping fairly tight lipped on this because they will be having to negotiate with each owner and don't want to prejudice the outcomes.