why air is used to inflate tyres?
and
why steel is use dto make railway lines
We know that air is highly compressible , so when it is filled in the tyres, it provides damping and adjusts easily to get the required foot print and required friction depending on the surface conditions. We can use gases for this purpose. But air is widely available without any cost so it is used.
Steel is used for railway lines because it has a high tensile strength and appropriate hardness.It has resistance to internal and surface cracks.