For those who are unfamiliar with the Thermite welding process, the red powder @ 1:31 is Foundry Sand which is used to seal the gaps around the mold. The grey powder @ 3:32 is Thermite which is a mixture of powdered Iron Oxide + powdered Aluminum. @ 4:25, the mold is preheated. @ 5:30, the Thermite contained inside the bucket is ignited. The high heat from the exothermic reaction produces molten iron which flows down into the mold. When the iron has solidified sufficiently, then the excess flash from the mold is ground off to make level and smooth the now repaired joint in the railway track.