I have often wondered about 485's fate myself. My questions were answered when I saw a photo of it after being removed from the pit. The right cylinder block was badly broken in the area of the piston valve. You could actually see the piston valve itself.
About the only way to repair a broken cylinder in this area would be to weld, or braze it, a very hit-or-miss proposition on oil soaked low quality cast iron. Combine that with the fact that the part that was missing was probably smashed into a hundred pieces, and it was it an area of the cylinder block that was very complicated, and the basic conclusion must be that the cylinder block is scrap.
Had this happened in the ‘30’s or ‘40’s, they could have gone to Baldwin for a replacement, but not in the ‘50’s. Add that to the declining fortunes of the n. g. and it isn’t much of a mystery.
It was certainly bad luck that the cylinder was damaged at all, without it being in about the worst possible place. Broken frames and boiler damage can be relatively easily repaired, but not broken cylinder blocks, especially in the area of the piston valve ports.