If there is gap →
When the material (railway track, concrete, steel, etc.) is heated it expands and when it is cooled it contracts. So the railway track or the bridge in summer season will be heated and so it tries to expand. So the track or the bridge will try to bend in different directions. But the track or the bridge in winter season will be cooled and so it tries to contracts. So the track or the bridge will try to straighten or separate themselves apart by cracking.
If there is no gap →
When the railway track or the bridge tries to expand the gap is filled. When the railway track or the bridge tries to contract the gap is increased some more. Thus the railway track or the bridge stays in their original direction.
Always the outside air contains some water in the invisible state (Gaseous State).When the cool drink or water bottle cools the outside air, the outside air gives some water in the visible state (Liquid State). The water from the outside air stays outside the bottle and looks like the bottle sweats.
There are seven colours in white light which is emitted by the sun. When the light travels through the atmosphere it scatters. That means the seven colour scatters. If a colour (Red) has long wavelength it scatters after traveling a long distance. If a colour (Violet ) has short wavelength it scatters after traveling a short distance. Blue colour has medium wavelength. When the sun is above our head (afternoon ) the distance in which the blue colour scatters is in our atmosphere. So we see the sky as blue in colour. When the sun is long (sunset ) the distance in which the red colour scatters also is in our atmosphere. So we see the sky near the sun as red in colour.