We can see that the windows of the bus, micro, and home are rectangular. Car windows are also mostly rectangle in shape though the partitions are angled. But, why do the plane and train have round shape?
A frequent traveller might have given some thought regarding why the window of an airplane is not rectangular in shape. The one who have seen and travelled by plane would surely know that every airplane they had travelled had a round window which may be a circle or an oval. There is not any exquisite reason behind this but there is actually fascinating scientific explanation behind this design of airplane windows. The windows of the airplane have not always been circular or of round shape or oval. It has been rectangular as well. But the rectangular window wound up making the plane crash. The round design is improvised later actually to keep the people and plane safe. In 1954, 56 passengers were killed when the two plane de Havilland Comet disintegrated and fell apart. It was the first commercial jetliner in the 1950s. 
Square windows happened to stress and crack due to being fixed with under pressure cabin. Whereas round window made those strain alleviate by dispensing it around the boundary of the window but the rectangle window were concentrated in the corner of a 90 degree rectangle window.
The reason behind stress is while the airplane takes off high into the sky, the atmospheric pressure outside declines more and more as the airplane gets higher. This creates the situation where the pressure inside the plane is greater than outside leading to pressure differentials creating rectangular glass fatigue and crack. According to a test by Royal Aircraft Establishment around 70 percent of the airplane stress was concentrated on the sharp angles of windows. So Later the rectangular windows were avoided for safety and round windows were incorporated.
Source:bgr, mentalfloss