My Grandfather told me,when I was young, of days when he worked on Electrifying the eastern cape.
The Company ,Eskom , would use the railway to transport the transformers and so required that the engineers also extend the railway at the same time so they spent many years supervising the laying of rail and so on …
During his time he also spoke of how they would weld the track together using Thermite , a mixture of half red iron oxide ( used to colour concrete) and half aluminum powder, and use a sparkler as a fuse, as welding rail track is not a simple affair, and the welding machines of the day couldn’t have managed it.
They used clay to make a mold below the container for the thermite , there was a gap between the tracks and the clay is molded so that the gap forms a cavity and then the container with the thermite is placed above and lit ..
the hot Liquid Iron then falls into the gap and fills it then cools .. forming a good bond
i don’t know if this is useful , as i haven’t tried it but it might be if you have no power and need to Macro weld something large …
I’m sure you could also do some simple casting with this method , there is a lot of labor involved in making powdered aluminum or iron oxide .. but I’m sure knowing this might be useful to some, one day, post shtf .
on editing this post i found this wiki article