In traditional railway construction, rails are laid down over sleepers, clamped to them and then fastened to one another by means of fishplates. These fishplates not only help the whole track structure to maintain its integrity, but also ensure that a certain (small) distance between rails is kept. This is due to thermal expansion.

All solids expand (increase in size) in a certain measure when subjected to high temperatures — and contract when cooled. This is a basic thermodynamic fact: molecules always vibrate and when heated, this vibration increases, slightly expanding the bulk of the material. When the main dimension of a solid is linear (like in rails) we talk of linear thermal expansion. This effect can be modeled with a constant coefficient for ambient temperatures. Rails are made of a steel which has a linear thermal expansion coefficient of around 12 µm/(m·K). That is: each meter of rail expands 0,012 millimeters for each degree of temperature increase.