Hint: Think of what materials railway tracks are generally made up of and arrive at how the material behaves under increasing/decreasing temperatures. In other words, we know that railway tracks are generally made of steel, which is a metal. Recall how metals change in structure when heated, and what would happen to the tracks if no gaps were left between rails.
Complete step by step answer:
We know that a railway track is a set of two parallelly placed rows of long pieces of steel that run over large distances. The uniform gap maintained between the rows of steel is called the gauge. It has additional structures like sleepers and ballast that help spread the load and impact that the train wheels impose on the ground.
It is of a general notion that metals expand on heating and contract on cooling. The extent of expansion or contraction depends on the coefficient of thermal expansion of the material which describes how the size of an object varies with changes in temperature.
Railway tracks are not made up of just one long piece of steel running continuously. They consist of many sections of steel placed next to each other. Two sections of rails are strategically placed at the joints such that a small gap is left between them. This is done to ensure that sufficient space is left to accommodate the thermal expansion of the rails during summers where the steel, owing to its metallic property absorbs heat from the atmosphere which increases the frequency and magnitude of the molecular motion within the material. This results in the production of energetic collisions which further forces the colliding molecules apart and causes the material to expand. If no space is left to house this expansion the track may become deformed and ultimately lead to rail buckling. This can lead to undesirable outcomes like derailing or other railway accidents.
Hence, the correct answer is option A.
Gaps give the space to the tracks to expand in summer heat.
Invar, a Ni-Fe alloy is known to have a uniquely low coefficient of thermal expansion, whereas Aluminium has one of the highest known coefficients of linear thermal expansion. Steel has a moderately high coefficient of linear thermal expansion.
It is important to remember that the coefficient of thermal expansion measures the fractional change in size per degree change in temperature but at a constant pressure.
In addition to atmospheric heat, another reason contributing to the heating up of the rails that is not specific to just summers would be frictional heating arising from the fast-moving wheels of the train over the track.