Final answer:
Train tracks expand during the summer season due to thermal expansion caused by an increase in temperature. Expansion joints are used to accommodate the expansion and prevent damage to the tracks.
Step-by-step explanation:
The expansion of train tracks during the summer season is due to thermal expansion, which is the change in size or volume of a given mass with temperature. When the temperature increases, the individual atoms in the tracks gain kinetic energy, causing them to move more vigorously and take up more space. This results in the tracks expanding in length and sometimes buckling if there are not enough expansion joints to accommodate the expansion.
An example of thermal expansion can be seen in the expansion of railroad tracks and roadways, which can buckle on hot days without sufficient expansion joints. Power lines also sag more in the summer due to thermal expansion. Expansion joints allow these structures to freely expand and contract with temperature changes to prevent damage.