Thermal expansion

In physics, thermal expansion is the likelihood of matter to change in volume in response to a change in temperature. When a substance is heated, its basic particles move around more quickly and by doing so generally maintain a greater average separation. Materials that contract with an increase in temperature are very uncommon; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature. 
Thermometers are an example of using thermal expansion. They contain a liquid which can only move in one direction (along the tube) when volume changes along with temperature.
Thermal expansion can become a problem for trains because it can cause the rails to buckle. On the rails they will have monitors so that if it does get abnormally high in temperature they can be warned and trains could be told to slow down to reduce the heat of friction. Sometimes on the inner parts of the rails they will be painted white to reflect the sun's hot rays so that it won't cause buckling.