Why does Railway Tracks expand and contract during temperature change ?

     Generally Rails are made up of steel. When steel is subjected to high temperature it expands but does not changes its shape. In solids atoms are very closely packed and when the solid is subjected to high temperature the molecules starts vibrating and the gap between the molecules increases so there will be expansion in the material. When the temperature starts decreasing, The molecules in the material starts getting close to each other so the material contracts. During Summer, the rails are subjected to high temperature and expands longitudinally but not laterally in the Winter the contracts due to low temperatures. These rails even expands and contracts during Day and Night regularly. The gap is maintained between the rails to expand freely. If this gap is not maintained the expansion is restricted and rails get bends. Due to this phenomenon, These bends may lead to accidents.