Rail steel is used to make rails for railway lines and for other uses such as tracks for moving equipment like cranes, transfer cars etc. Heavier rails carry heavier and faster trains on the tracks. This steel must be hard wearing and resistant to cracking. This is achieved by careful choice of composition of steel and controlling the way the hot rail is cooled. Before the development of heat treatment processes, the tensile strength and hardness of naturally cooled rails were controlled by the chemical composition of the steel. Both of these properties of steel depend upon the proportion of carbon and manganese.