The correct answer is option 1: Aluminium. In the joining of railway tracks, a process called thermite welding is used. Thermite welding is a type of exothermic reaction that produces intense heat to join metal parts together. In this process, iron oxide, also known as rust, is combined with a reducing agent, which is aluminium powder. When the iron oxide reacts with aluminium, a highly exothermic reaction occurs, resulting in the production of liquid iron and aluminium oxide slag.

This reaction is highly favored due to the large negative enthalpy change ([REPLACEMENT]H) and is thermodynamically spontaneous. The high temperatures reached through this reaction allow for the melting and joining of the railway tracks. The liquid iron fills the gap between the tracks and solidifies, creating a strong and durable welded joint.

It is important to note that the other options mentioned in the question, Zinc, Copper, and Tin, do not react with iron oxide in the same way as Aluminium does. Therefore, they are not used in the joining of railway tracks through thermite welding.