This downward train has just made a careful and safe stop to check the brakes. If you look carefully, you can see a railroader walking along the 2nd tank car
The train has 23 tank cars full of sulfuric acid. That is a very heavy train considering the grade, and it goes down very slowly. I saw it stopping twice, probably for cooling the brakes.
Potrerillos is not far away. It is a matter of a few kilometers. But the train has left the yard 27 minutes ago. Between Potrerillos and Montandon, the gradient is a dangerous 3.9%. Engineers are extra cautious so as to keep their train under control.
Additional information kindly provided by Mel Turner (from Australia): "The Potrerillos Refinery treats concentrate produced mainly from sulphated ores, so sulphuric acid is a byproduct of the smelting process. In Chile low grade oxided ores are processed via the solvent extraction electrowinning (SX/EW) method, using sulphuric acid as the solvent. This process results in the thin annode sheets you see on the FCAB flatcars. Potrerillos is unique in Chile in that it is the only refinary where the concentrate is moved against the grade. In 1999 CODELCO installed a US$100 million sulphuric acid plant, which captured 73% of SO2 emissions, there have since been 2 upgrades to increase the capture rate, this plant also produces arsenic, another by-product of the refining process. So 150k tonnes of acid per year is shipped from Potrerillos to Mina Franke, some is shipped to Salvador to feed their SX/EW leaching pads, and the balance goes down to the tank farm at Barquito from where it is moved by ship to other CODELCO operations and customers."