What is thermit reaction? How is it used to join the railway tracks or cracked machine parts?

(N/A) The reaction which is highly exothermic and in which metal is produced in the molten state is known as the thermit reaction.
Consider the following displacement reaction:
$Fe_{2}O_{3}(s) + 2Al(s) \xrightarrow{\text{Heat}} Al_{2}O_{3}(s) + 2Fe(l) + \text{Heat}$
In this reaction,when iron $(III)$ oxide $(Fe_{2}O_{3})$ is heated with aluminium powder $(Al)$,the amount of heat evolved is so large that the iron $(Fe)$ metal is produced in a molten state.
This molten iron flows into the gaps of the railway tracks or cracked machine parts and,upon cooling,solidifies to join them together.