I've returned to this puzzling structure and spent most of an afternoon measuring it and generally studying it.
Firstly I must withdraw my previous sarcastic comment about the roof being the work of drunken restorers. It might look like that but it isn't, it's an old roof, not only is there a wider variety of plants growing up there than anywhere nearby but there is a full grown tree rooted in the roof!! You can age a hedge by counting the number of different species growing in it, I sure the same is true of a roof. The tree appears to have been growing for about as long as many of the trees on that level so I guess they all took root about the same time after abandonment of the level. (The OS maps of about 1900 do show the odd tree here and there but I am comparing with the trees that could not have been there when the level was working.)
The internal dimensions are 5ft wide, 4ft 4in high, 12 ft deep, a bit higher than I previously estimated but still very low, it might have been intended to be 4ft 6in high as the path across the front has intruded just a bit. It's slightly lower at the back but that is probably because the rear wall has bulged out a bit and is now a weak point in the structure and may blow in the next couple of years. All exterior walls are 3ft thick (the nearby gwalia are 2ft 6in) and the roof is 3ft 6in thick at the quarry side tapering to 3ft at the drumhouse side. The recess in the rear wall is 2ft wide, 1ft 9in high, and 1ft 9in deep.
I had a good look at the floor (after moving the droppings of various species) it seems to be an earth floor with randomly scattered pieces of slate. I found no evidence of rails or fixings of any kind. It is possible that I wasn't seeing the original floor, years of leaf mold plus a little intruded earth (plus droppings) and the random scattering of bits of slate (like every where nearby) could be hiding something but I wasn't excavating, just using my eyes! The floor is about 1ft 6in - 2ft below the level of the rails that ran from the quarry behind this structure past the gwalia to the drumhouse, and the entrance to the structure is very close to the edge of the work-area so I stick to my previous comment about the improbability of it being a shelter for a loco of any kind.
The construction is interesting, it's subtly different to all the other structures nearby. There are no large blocks of slate, in fact much of the slate appears to be very poor quality, many pieces have several veins of pyrites in them. I'd even suggest that many of the pieces might better be classed as overburden rather than proper slate. Its difficult to describe the difference but I got the strong impression the the slates came from the preparation of the level rather than the working of the level. I could not find any toolmarks on any of the slates this is different to nearby structures. JohnnearCfon commented that there appeared to be three phases of construction and indeed the photo does give that impression. However looking at the actual structure I do not think this is the case, I think its the side-effect of a repair. The wooden lintel above the doorway cannot be original, its relatively modern timber which woodworm has not touched, all the remainder of the roof is supported on tree branches in which the woodworm have had very many feasts. I think the insertion of the replacement lintel has produced an optical delusion.
Its clear from the bulge in the rear wall that earth has been used as a packing material in the depth of the wall as in many dry stone walls, a feature I have not noticed elsewhere in the quarry. Also I'm fairly sure that this structure is built on solid ground and not the built-up ground which makes up most of the work-area. My strong impression is that this structure was built in the earliest days of this level possibly/probably before the retaining wall and the built-up ground under the gwalia and the drumhouse.
There is a similarly located structure on the V3 level, however its much bigger, internal 5ft wide(same), 5ft 6in high(bigger), 18ft deep(much bigger), walls are 3ft thick(same), roof is 2ft 6in thick(thinner) and flat. Another obvious difference this building had internal plaster the one above had not. Also its built of better quality, better shaped slate and on the same level as the rest of the work-area. In the past I'd always tended to assumed (without evidence) that this was a loco overnight shelter.
~~~ The future is not what it used to be ~~~