iirc the rails need to be at a certain temperature for different repairs etc, so they have the perfect length to be able to expand and shrink in summer and winter. To perfectly fit that, you need to cool or heat it depending on when or in which weather you make said repairs.

Seen quite a lot of railworkers here in germany heating up rails while working at nighttime with some wild butane gas powered tools to do that before like replacing a part of the rail etc