In the particular case of moving the spoil material, some sort of conveyor system would seem more appropriate than a narrow gauge railway - but I'm sure they would have considered that too.
Yes but it is not just for use in excavation. I mentioned the movement of materials so that includes taking materials onto the site as well as taking them away. One would only need a digger at one end as the spoil could be tipped on the dump area. All this kind of thing was used nearly 200 years ago-nothing new or difficult about it. I agree that it would require a rethink of how the work is carried out as people are not used to using a railway onsite these days. However if it were planned and people were trained then with some flexibility to could be made to work and in this case probably work more efficiently. A conveyor would be better but the site is too constrained for it to be useful. A linear project such as that with vehicles and materials moving along that corridor is surely ideal for railway use but I rather doubt if it was considered. They are in regular use in conjunction with TBMs.
I a previous job I was responsible for an internal operation that made about 200,000te per year of soil, and stored and despatched 150,000te per year of a lime product.
All movement of material was by dumper truck, even though the production and storage locations were more or less fixed. In the case of the soil it got moved several times to go through a drying, blending, conditioning and storage process. We probably moved close to 1,000,000te of material internally every year.
An internal narrow gauge railway wouldn't even have been considered. I did start looking at quarry-style conveyors which are relatively cheap and flexible but it didn't really stack up due to the fact that the operations only took place for 6 months of the year. Hiring a lot of dumper trucks was cheaper, easier to manage and more flexible.
One of the sites did manage to put a conveyor in to move the lime product to the storage area some half a mile away. But that took years of persuasion and only got approved through very careful recording of all the lost primary production when dumpers were enable to keep the lime hopper empty (and that was usually because the product was sticking in the hopper rather than the dumper solution not being good enough).
I imagine a lot of the same would be true for a linear site such as this project. In fact having a constrained site probably makes conveyors and railways less suitable, as it's sometimes necessary to change where you've planned to put spoil.