Why does the rail length decrease during cooling?

It is known that all bodies are composed of atoms and molecules, between which there is space. When heated, the gaps between the particles become smaller, in accordance with this, the linear dimensions of the body decrease. Therefore, the length of the rail becomes shorter as it cools.
Comment. There should be space between the rails, since as the temperature rises (summer heat), the rail becomes longer and will bend if there is no space between the rails.