Well... as we have been restoring the SN engine house, we know a few things...
Yes the walls and ceiling were originally decked over, but a lot was removed over the years, some to be able to reach the rafters for structural repairs. Thou decked over, there was no insulation in the walls or ceiling. So I can verify that that place was very cold in the winter.
Now during the restoration, there is insulation in the exterior walls, and all the structural issues have been addressed and repaired, most of the old siding has been replaced over Decking material, to look as old as it is, and a new corrugated roof has been added. All the windows were replaced with new custom created windows to exactly match the originals.
The foundations inside and out has been restored, and strengthened. The next phase ( when and if funding is procured) will be re-building the floors, and re-decking the walls and ceiling. And then the creation of a restoration workshop in the original rear machine shop area, to facilitate the maintenance of all the rail cars in the collection, and restoration of historic cars.