This is a good example of fusion of rails using the alumino-thermit process. Here a prefabricated mold is placed in between the rails and the whole assembly is pre-heated. A mixture of aluminium and iron oxide is ignited using magnesium, which results...

This is a good example of fusion of rails using the alumino-thermit process. Here a prefabricated mold is placed in between the rails and the whole assembly is pre-heated. A mixture of aluminium and iron oxide is ignited using magnesium, which results in an strongly exothermic reaction in which oxygen from iron is exchanged to aluminium.

Fe2O3 + 2Al -> 2Fe + Al2O3 + Heat

The molten iron is now poured in the mold which has a sprue (from where the hot metal is poured), mold cavity containing the rails to be fused and a runner (from where the molten metal rises up to indicate the filling level in the mold). The cylindrical structure that you see in the image is either a sprue or a runner, and most likely in my opinion a runner. An identical cylindrical portion should also exist on the other side with a basin which acts as sprue which receives the metal. Hot metal is NOT poured in the gap between the rails directly.

Lastly, this interesting technology is in fact very old and is called the "Goldschmidt reaction" or "Goldschmidt process" after the German chemist Hans Goldschmidt, who discovered in 1893.

Looks like molten steel is poured in the gap and extra piece chopped off.
What are the charactesitics of this steel to ensure uniform expansion / contraction with temperature changes. Do they melt some existing rails an pour it in the gap? thanks ...

Looks like molten steel is poured in the gap and extra piece chopped off.
What are the charactesitics of this steel to ensure uniform expansion / contraction with temperature changes. Do they melt some existing rails an pour it in the gap? thanks for this interesting piece of IR technology....