I was on it in Mexico around 20 years ago, it was a "big foot", standard gauged. SP apparently knew the end was near so ordered it with standard gauge truck frames for re-use or re-sale. They probably only needed it for 3-4 years to get their investment back, it lasted 6. The two at Sumpter have narrow gauge truck frames as most (all?) the other other 50 tons, good for 36-42". There was also an 80 ton center cab, and an xSP Alco S-6, all three looked like they had not been used in a while. The rail I saw around there was real small - maybe 45#.
The copper company was privitized in the 1990's, had some labor problems, and did an enviromental cleanup. With the price of scrap, my guess is that things would not look good for its survival, but the info I found also suggested the smelter was using rail again.
The answer to your question is a definate maybe. A SPng historian I am in contact with claims that it does still exist and there is a group he is involved with persuing returning it to the US. Keep your fingers crossed!