Bridges, roads, and railway lines are made in sections separated by rubber strips or gaps. Why?

A rail track made of steel having length 10 m is clamped on a railway line at its two ends on a summer day due to rise in temperature by 20^@C, it is deformed as shown in fig. Find x (displacement of the centre) if alpha_(steel) = 1.2 xx 10^(-5)//^@C.

Two persons stand near a railway line separated by a distance 2 km. A detonator is exploded by the first person. The second person 2 km away hears two sounds, one from the sound that has travelling through the rail and the other through the air. What is the interval between them? Given Young's modulus of steel = 2 xx 10^11 N//m^2. Density of steel = 8000 kg/m ""^3. Density of air = 1.29 kg/m ""^3. Atmospheric pressure =1 xx 10^(2) N//m^2. y for air = 1.4

Small gaps are left between railway lines because :

The surfaces of the train wheels and railway tracks are made smooth ". Why ?